[NFC] Use isX86() instead of getArch()

Summary: This is a clean up for https://reviews.llvm.org/D72247.

Reviewers: MaskRay, craig.topper, jhenderson

Reviewed By: MaskRay

Subscribers: hiraditya, rupprecht, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D72320