[Mangler] Mangle aliases to fastcall/vectorcall functions correctly

These aliases are produced by MergeFunctions and need to be mangled according to the calling convention of the function they are pointing to instead of defaulting to the C calling convention.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D120382

llvm/lib/IR/Mangler.cpp

@@ -144,7 +144,7 @@ void Mangler::getNameWithPrefix(raw_ostream &OS, const GlobalValue *GV,
 
   // Mangle functions with Microsoft calling conventions specially.  Only do
   // this mangling for x86_64 vectorcall and 32-bit x86.
-  const Function *MSFunc = dyn_cast<Function>(GV);
+  const Function *MSFunc = dyn_cast_or_null<Function>(GV->getAliaseeObject());
 
   // Don't add byte count suffixes when '\01' or '?' are in the first
   // character.

llvm/test/CodeGen/X86/fastcall-correct-mangling.ll

@@ -31,3 +31,7 @@ define private x86_fastcallcc void @dontCrash() {
 ; CHECK64-LABEL: {{^}}.LdontCrash:
   ret void
 }
+
+@alias = alias void(i64, i8, i8, i16), void(i64, i8, i8, i16)* @func
+; CHECK32-LABEL: {{^}}.set @alias@20, @func@20
+; CHECK64-LABEL: {{^}}.set alias, func