[llvm] X86: Avoid using isArch64Bit for 64-bit checks (PR #157412)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> We have x86_64 analogues in downstream code. It will make code complicated to check a couple of them each time.

Then there should be an isX86_64 helper in Triple like other targets do 

https://github.com/llvm/llvm-project/pull/157412