[WebAssembly] Added basic support for if/else/end_if in MC layer.

[aardappel]

Summary

These instructions are currently unused in our backend, but for
completeness it is good to support them, so they can be used with
the assembler in hand-written code.

Tests are very basic, signature support missing much like other blocks.

[aheejin]

LGTM. It looks like the change depends on D55797, so I added it as a parent revision.

lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

while (Lexer.isNot(AsmToken::EndOfStatement)) {
       }
     }
     Parser.Lex();
 
     // Block instructions require a signature index, but these are missing in
     // assembly, so we add a dummy one explicitly (since we have no control
     // over signature tables here, we assume these will be regenerated when
     // the wasm module is generated).
-    if (BaseName == "block" || BaseName == "loop" || BaseName == "try") {
+    if (BaseName == "block" || BaseName == "loop" || BaseName == "try" ||
+        BaseName == "if") {
       Operands.push_back(make_unique<WebAssemblyOperand>(
           WebAssemblyOperand::Integer, NameLoc, NameLoc,
           WebAssemblyOperand::IntOp{-1}));
     }
     return false;
   }
 
   void onLabelParsed(MCSymbol *Symbol) override {

lib/Target/WebAssembly/WebAssemblyInstrControl.td

 // This is technically a control-flow instruction, since all it affects is the
 // IP.
 defm NOP : NRI<(outs), (ins), [], "nop", 0x01>;
 
 // Placemarkers to indicate the start or end of a block or loop scope.
 // These use/clobber VALUE_STACK to prevent them from being moved into the
 // middle of an expression tree.
 let Uses = [VALUE_STACK], Defs = [VALUE_STACK] in {
-defm BLOCK     : NRI<(outs), (ins Signature:$sig), [], "block   \t$sig", 0x02>;
-defm LOOP      : NRI<(outs), (ins Signature:$sig), [], "loop    \t$sig", 0x03>;
+defm BLOCK : NRI<(outs), (ins Signature:$sig), [], "block   \t$sig", 0x02>;
+defm LOOP  : NRI<(outs), (ins Signature:$sig), [], "loop    \t$sig", 0x03>;
 
-// END_BLOCK, END_LOOP, and END_FUNCTION are represented with the same opcode in
-// wasm.
+defm IF : I<(outs), (ins Signature:$sig, I32:$cond),
+            (outs), (ins Signature:$sig),
+            [], "if    \t$sig, $cond", "if    \t$sig", 0x04>;
+defm ELSE : NRI<(outs), (ins), [], "else", 0x05>;
+
+// END_BLOCK, END_LOOP, END_IF and END_FUNCTION are represented with the same
+// opcode in wasm.
 defm END_BLOCK : NRI<(outs), (ins), [], "end_block", 0x0b>;
 defm END_LOOP  : NRI<(outs), (ins), [], "end_loop", 0x0b>;
+defm END_IF    : NRI<(outs), (ins), [], "end_if", 0x0b>;
 let isTerminator = 1, isBarrier = 1 in
 defm END_FUNCTION : NRI<(outs), (ins), [], "end_function", 0x0b>;
 } // Uses = [VALUE_STACK], Defs = [VALUE_STACK]
 
 multiclass RETURN<WebAssemblyRegClass vt> {
   defm RETURN_#vt : I<(outs), (ins vt:$val), (outs), (ins),
                       [(WebAssemblyreturn vt:$val)],
                       "return  \t$val", "return", 0x0f>;

test/MC/WebAssembly/basic-assembly.s

.LBB0_2:
     i32.const 1
     br 2
     end_block            # second entry jumps here.
     i32.const 2
     br 1
     end_block            # default jumps here.
     i32.const 3
     end_block            # "switch" exit.
-    #if                  # These are not in tablegen defs yet..
-    #if
-    #end_if
-    #else
-    #end_if
+    if
+    if
+    end_if
+    else
+    end_if
     f32x4.add
     # Test correct parsing of instructions with / and : in them:
     # TODO: enable once instruction has been added.
     #i32x4.trunc_s/f32x4:sat
     i32.trunc_s/f32
     try
 .LBB0_3:
     i32.catch   0

 # CHECK-NEXT:      i32.const 1
 # CHECK-NEXT:      br 2                # 2: down to label2
 # CHECK-NEXT:      end_block           # label4:
 # CHECK-NEXT:      i32.const 2
 # CHECK-NEXT:      br 1                # 1: down to label2
 # CHECK-NEXT:      end_block           # label3:
 # CHECK-NEXT:      i32.const 3
 # CHECK-NEXT:      end_block           # label2:
-# DONT-CHECK-NEXT:      if
-# DONT-CHECK-NEXT:      if
-# DONT-CHECK-NEXT:      end_if
-# DONT-CHECK-NEXT:      else
-# DONT-CHECK-NEXT:      end_if
+# CHECK-NEXT:      if
+# CHECK-NEXT:      if
+# CHECK-NEXT:      end_if
+# CHECK-NEXT:      else
+# CHECK-NEXT:      end_if
 # CHECK-NEXT:      f32x4.add
 # CHECK-NEXT:      i32.trunc_s/f32
 # CHECK-NEXT:      try
 # CHECK-NEXT:  .LBB0_3:
 # CHECK-NEXT:      i32.catch   0
 # CHECK-NEXT:  .LBB0_4:
 # CHECK-NEXT:      catch_all
 # CHECK-NEXT:  .LBB0_5:
 # CHECK-NEXT:      end_try
 # CHECK-NEXT:      get_global  __stack_pointer@GLOBAL
 # CHECK-NEXT:      end_function
 
 # CHECK:           .globaltype  __stack_pointer, i32