[llvm] [X86] Clear regcall arg aliases in dynamic CSR masks (PR #225262)

[Demetrios Chiuratto Agourakis via llvm-commits]

agourakis82 wrote:

@HelloWorldU the return-register case is now covered by `test_ret_subreg` (10th struct field returned in R14D; R14 must be cleared from CustomRegMask).

Re the Copilot note: these calls lower to `CALL64m` (GOT-indirect, non-dso_local declarations), not `CALL64pcrel32`. I verified on current main: the test passes with the fix and fails on both `CHECK-NOT: $r14,` lines without it.

https://github.com/llvm/llvm-project/pull/225262