[clang] [clang][SPIR-V] Always add convervence intrinsics (PR #88918)

[Nathan Gauër via cfe-commits]

Keenuts wrote:

 Hi all, rebased on main, and addressed the comments.
 This commits changes the register order on SPIR-V vs DXIL, which required me to fix the mad+lerp intrinsic tests. Should be NFC, just storing the register name in a CHECK variable.

https://github.com/llvm/llvm-project/pull/88918