[WebAssembly] Assembler: support .int16/32/64 directives.

lib/Target/WebAssembly/AsmParser/WebAssemblyAsmParser.cpp

if (DirectiveID.getString() == ".local") {
       SmallVector<wasm::ValType, 4> Locals;
       if (parseRegTypeList(Locals))
         return true;
       TOut.emitLocal(Locals);
       CurrentState = FunctionLocals;
       return expect(AsmToken::EndOfStatement, "EOL");
     }
 
-    if (DirectiveID.getString() == ".int8") {
+    if (DirectiveID.getString() == ".int8" ||
+        DirectiveID.getString() == ".int16" ||
+        DirectiveID.getString() == ".int32" ||
+        DirectiveID.getString() == ".int64") {
       if (CheckDataSection()) return true;
-      int64_t V;
-      if (Parser.parseAbsoluteExpression(V))
-        return error("Cannot parse int8 constant: ", Lexer.getTok());
-      // TODO: error if value doesn't fit?
-      Out.EmitIntValue(static_cast<uint64_t>(V), 1);
+      const MCExpr *Val;
+      SMLoc End;
+      if (Parser.parseExpression(Val, End))
+        return error("Cannot parse .int expression: ", Lexer.getTok());
+      size_t NumBits = 0;
+      DirectiveID.getString().drop_front(4).getAsInteger(10, NumBits);
+      Out.EmitValue(Val, NumBits / 8, End);
       return expect(AsmToken::EndOfStatement, "EOL");
     }
 
     if (DirectiveID.getString() == ".asciz") {
       if (CheckDataSection()) return true;
       std::string S;
       if (Parser.parseEscapedString(S))
         return error("Cannot parse string constant: ", Lexer.getTok());

test/MC/WebAssembly/basic-assembly.s

.LBB0_5:
     end_function
 
     .section    .rodata..L.str,"",@
     .hidden     .L.str
     .type       .L.str,@object
 .L.str:
     .int8       'H'
     .asciz      "ello, World!"
-    .size       .L.str, 14
+    .int16      1234
+    .int64      5000000000
+    .int32      2000000000
+    .size       .L.str, 28
 
     .ident      "clang version 9.0.0 (trunk 364502) (llvm/trunk 364571)"
     .globaltype __stack_pointer, i32
 
 # CHECK:           .text
 # CHECK-LABEL: test0:
 # CHECK-NEXT:      .functype   test0 (i32, i64) -> (i32)
 # CHECK-NEXT:      .eventtype  __cpp_exception i32

 # CHECK-NEXT:      global.get  __stack_pointer
 # CHECK-NEXT:      end_function
 
 # CHECK:           .section    .rodata..L.str,"",@
 # CHECK-NEXT:      .hidden     .L.str
 # CHECK-NEXT:  .L.str:
 # CHECK-NEXT:      .int8       72
 # CHECK-NEXT:      .asciz      "ello, World!"
+# CHECK-NEXT:      .int16       1234
+# CHECK-NEXT:      .int64       5000000000
+# CHECK-NEXT:      .int32       2000000000
 
 # CHECK:           .globaltype __stack_pointer, i32