[WebAssembly] Call signExtend to get sign extended register [NFCI]

[jbhateja]

Summary

While analyzing Bug 35064 - Crash in WebAssembly register allocation.
observed this incorrect call.

[sunfish]

Good catch; this change is obviously correct. Would you mind adding a testcase?

[jbhateja]

• Adding a test for signext of function arguments.

lib/Target/WebAssembly/WebAssemblyFastISel.cpp

unsigned WebAssemblyFastISel::getRegForUnsignedValue(const Value *V) {
   MVT::SimpleValueType From = getSimpleType(V->getType());
   MVT::SimpleValueType To = getLegalType(From);
   return zeroExtend(getRegForValue(V), V, From, To);
 }
 
 unsigned WebAssemblyFastISel::getRegForSignedValue(const Value *V) {
   MVT::SimpleValueType From = getSimpleType(V->getType());
   MVT::SimpleValueType To = getLegalType(From);
-  return zeroExtend(getRegForValue(V), V, From, To);
+  return signExtend(getRegForValue(V), V, From, To);
 }
 
 unsigned WebAssemblyFastISel::getRegForPromotedValue(const Value *V,
                                                      bool IsSigned) {
   return IsSigned ? getRegForSignedValue(V) :
                     getRegForUnsignedValue(V);
 }