[Headers][X86] Allow pmuludq/pmuldq to be used in constexpr

[RKSimon]

Currently both of these are expanded in CodeGenFunction::EmitX86BuiltinExpr - there are a few options on what we do:

1 - add pmuldq/pmuludq handling to VectorExprEvaluator::VisitCallExpr similar to #152524
2 - remove the pmuldq/pmuludq / EmitX86Muldq expansion and replace it with a generic implementation in the headers directly - not sure how much worse the non-optimised expansion will be if we did this.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Simon Pilgrim (RKSimon)

Currently both of these are expanded in CodeGenFunction::EmitX86BuiltinExpr - there are a few options on what we do:

1 - add pmuldq/pmuludq handling to VectorExprEvaluator::VisitCallExpr similar to #152524
2 - remove the pmuldq/pmuludq / EmitX86Muldq expansion and replace it with a generic implementation in the headers directly - not sure how much worse the non-optimised expansion will be if we did this.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Currently both of these are expanded in CodeGenFunction::EmitX86BuiltinExpr - there are a few options on what we do:

1 - add pmuldq/pmuludq handling to VectorExprEvaluator::VisitCallExpr similar to #152524
2 - remove the pmuldq/pmuludq / EmitX86Muldq expansion and replace it with a generic implementation in the headers directly - not sure how much worse the non-optimised expansion will be if we did this.

[pedroclobo]

I can work on this.