[llvm] X86: Fix asserting on bfloat argument/return without sse2 (PR #93146)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

LGTM (mainly just to stop the assert), but I'd prefer @phoebewang replies as well

https://github.com/llvm/llvm-project/pull/93146