[LLVM][X86] Add f80 support for ct.select

Add special handling for x86_fp80 types in CTSELECT lowering by splitting
them into three 32-bit chunks, performing constant-time selection on each
chunk, and reassembling the result. This fixes crashes when compiling
tests with f80 types.

Also updated ctselect.ll to match current generic fallback implementation.

Author: wizardengineer