Bug 1710024 - Add comments about input reuse, r=yury

Add some comments about input reuse on arm64 so that we're not
confused in the future as to why this is OK.

Differential Revision: https://phabricator.services.mozilla.com/D117917

js/src/jit/Lowering.cpp

@@ -1378,6 +1378,8 @@ void LIRGenerator::visitMinMax(MMinMax* ins) {
       MOZ_CRASH();
   }
 
+  // Input reuse is OK (for now) even on ARM64: floating min/max are fairly
+  // expensive due to SNaN -> QNaN conversion, and int min/max is for asm.js.
   defineReuseInput(lir, ins, 0);
 }
 
@@ -2419,6 +2421,8 @@ void LIRGenerator::visitWasmExtendU32Index(MWasmExtendU32Index* ins) {
   MOZ_ASSERT(input->type() == MIRType::Int32);
   MOZ_ASSERT(ins->type() == MIRType::Int64);
 
+  // Input reuse is OK even on ARM64 because this node *must* reuse its input in
+  // order not to generate any code at all, as is the intent.
   auto* lir = new (alloc()) LWasmExtendU32Index(useRegisterAtStart(input));
   defineReuseInput(lir, ins, 0);
 #else
@@ -2432,6 +2436,8 @@ void LIRGenerator::visitWasmWrapU32Index(MWasmWrapU32Index* ins) {
   MOZ_ASSERT(input->type() == MIRType::Int64);
   MOZ_ASSERT(ins->type() == MIRType::Int32);
 
+  // Input reuse is OK even on ARM64 because this node *must* reuse its input in
+  // order not to generate any code at all, as is the intent.
   auto* lir = new (alloc()) LWasmWrapU32Index(useRegisterAtStart(input));
   defineReuseInput(lir, ins, 0);
 #else