[WebAssembly] Unstackify registers with no uses in ExplicitLocals

There are cases we end up removing some intructions that use stackified
registers after RegStackify. For example,

```wasm
bb.0:
  %0 = ...    ;; %0 is stackified
  br_if %bb.1, %0
bb.1:
```

In this code, br_if will be removed in CFGSort, so we should unstackify
%0 so that it can be correctly dropped in ExplicitLocals.

Rather than handling this in case-by-case basis, this PR just
unstackifies all stackifies register with no uses, so that they can be
correctly dropped.

Fixes #149097.

Author: aheejin

llvm/lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

@@ -256,9 +256,17 @@ bool WebAssemblyExplicitLocals::runOnMachineFunction(MachineFunction &MF) {
 
   // Precompute the set of registers that are unused, so that we can insert
   // drops to their defs.
+  // And unstackify any stackified registers that don't have any uses, so that
+  // they can be dropped later. This can happen when transformations after
+  // RegStackify removes instructions that use stackified registers.
   BitVector UseEmpty(MRI.getNumVirtRegs());
-  for (unsigned I = 0, E = MRI.getNumVirtRegs(); I < E; ++I)
-    UseEmpty[I] = MRI.use_empty(Register::index2VirtReg(I));
+  for (unsigned I = 0, E = MRI.getNumVirtRegs(); I < E; ++I) {
+    Register Reg = Register::index2VirtReg(I);
+    if (MRI.use_empty(Reg)) {
+      UseEmpty[I] = true;
+      MFI.unstackifyVReg(Reg);
+    }
+  }
 
   // Visit each instruction in the function.
   for (MachineBasicBlock &MBB : MF) {

llvm/test/CodeGen/WebAssembly/removed-terminator.ll

@@ -0,0 +1,14 @@
+; RUN: llc -O0 < %s
+
+target triple = "wasm32-unknown-unknown"
+
+define void @test(i1 %x) {
+  %y = xor i1 %x, true
+  ; This br_if's operand (%y) is stackified in RegStackify. But this terminator
+  ; will be removed in CFGSort after that. We need to make sure we unstackify %y
+  ; so that it can be dropped in ExplicitLocals.
+  br i1 %y, label %exit, label %exit
+
+exit:
+  ret void
+}