[WebAssembly] Make rethrow instruction take a target BB argument

[aheejin]

Summary

This patch changes the rethrow instruction to take a BB argument in LLVM
backend, like br and br_ifs. This BB is a target catch BB the
rethrow instruction unwinds to. This BB argument will be converted to an
relative depth immediate at the end of CFGStackify pass, as in the same
way of branches.

RETHROW_TO_CALLER is a codegen-only instruction that should be used when
a rethrow instruction does not have an unwind destination BB, i.e., it
should rethrow to its caller function.

[aheejin]

• Printed name change

[aheejin]

• Comment for RETHROW_TO_CALLER

[dschuff]

LGTM; I think this model definitely makes more sense in CFG-based IR.

[aheejin]

Changed to the new syntax of D48183 (rL334985)

[aheejin]

Changed to the new syntax. PTAL

[dschuff]

still LGTM

lib/Target/WebAssembly/WebAssemblyInstrControl.td

 // Throwing an exception: throw / rethrow
 let isTerminator = 1, hasCtrlDep = 1, isBarrier = 1 in {
 def THROW_I32 : I<(outs), (ins i32imm:$tag, I32:$val),
                   [(int_wasm_throw imm:$tag, I32:$val)], "throw   \t$tag, $val",
                   0x08>;
 def THROW_I64 : I<(outs), (ins i32imm:$tag, I64:$val),
                   [(int_wasm_throw imm:$tag, I64:$val)], "throw   \t$tag, $val",
                   0x08>;
-def RETHROW : I<(outs), (ins i32imm:$rel_depth), [], "rethrow \t$rel_depth",
-                0x09>;
+def RETHROW : I<(outs), (ins bb_op:$dst), [], "rethrow \t$dst", 0x09>;
+let isCodeGenOnly = 1 in
+def RETHROW_TO_CALLER : I<(outs), (ins), [], "rethrow_to_caller">;
 } // isTerminator = 1, hasCtrlDep = 1, isBarrier = 1
 
 // Region within which an exception is caught: try / end_try
 let Uses = [VALUE_STACK], Defs = [VALUE_STACK] in {
 def TRY     : I<(outs), (ins Signature:$sig), [], "try     \t$sig", 0x06>;
 def END_TRY : I<(outs), (ins), [], "end_try", 0x0b>;
 } // Uses = [VALUE_STACK], Defs = [VALUE_STACK]