[X86] Teach X86SelectionDAGInfo::EmitTargetCodeForMemcpy about GNUX32

Summary:
In GNUX23, is64BitMode returns true, but pointers are 32-bits. So we shouldn't copy pointer values into RSI/RDI since the widths don't match.

Fixes PR38865 despite what the title says. I think the llvm_unreachable in the copyPhysReg code tricked the optimizer and made the fatal error trigger.

Reviewers: rnk, efriedma, MatzeB, echristo

Reviewed By: efriedma

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51893

llvm-svn: 342015