[clang] [X86][RFC] Refactor the SSE intrinsics constexpr tests to simplify future expansion (PR #112578)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

I've added builtin_test_helpers.h header to share the comparison code - if there's no objections I'll push this tomorrow.

https://github.com/llvm/llvm-project/pull/112578