[LLVM][X86] Add native ct.select support for X86 and i386
Add native X86 implementation with CMOV instructions and comprehensive tests:
- X86 ISelLowering with CMOV for x86_64 and i386
- Fallback bitwise operations for i386 targets without CMOV
- Post-RA expansion for pseudo-instructions
- Comprehensive test coverage:
- Edge cases (zero conditions, large integers)
- i386-specific tests (FP, MMX, non-CMOV fallback)
- Vector operations
- Optimization patterns
The basic test demonstrating fallback is in the core infrastructure PR.
