[wasm] Enfore valid register for SignExtendWord8ToInt32.

On ia32, the instruction selector uses movsx_b to compile the wasm
SignExtendWord8ToInt32 instruction. movsx_b requires a byte register
as input. However, not all allocatable registers on ia32 are. As we
cannot currently express constraints on subsets of registers, this
change now forces the input to movsx_b into eax.

Bug: chromium:919572
Change-Id: I39bd391974954ec9044940c3164398109eb78908
Reviewed-on: https://chromium-review.googlesource.com/c/1400409Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58622}

src/compiler/backend/ia32/instruction-selector-ia32.cc

@@ -185,8 +185,12 @@ namespace {
 void VisitRO(InstructionSelector* selector, Node* node, ArchOpcode opcode) {
   IA32OperandGenerator g(selector);
   InstructionOperand temps[] = {g.TempRegister()};
-  selector->Emit(opcode, g.DefineAsRegister(node), g.Use(node->InputAt(0)),
-                 arraysize(temps), temps);
+  Node* input = node->InputAt(0);
+  // We have to use a byte register as input to movsxb.
+  InstructionOperand input_op =
+      opcode == kIA32Movsxbl ? g.UseFixed(input, eax) : g.Use(input);
+  selector->Emit(opcode, g.DefineAsRegister(node), input_op, arraysize(temps),
+                 temps);
 }
 
 void VisitRR(InstructionSelector* selector, Node* node,