[X86] Fold vgf2p8affineqb XOR with splat constant into immediate (#179103)

The vgf2p8affineqb instruction performs an affine transformation on each
byte and then XORs the result with an 8-bit immediate operand. When this
instruction is followed by a standalone XOR with a splatted constant,
LLVM currently generates extra instructions instead of folding the
constant into the instruction's immediate.
This PR adds a DAG combine optimization that detects the pattern
vgf2p8affineqb(x, m, imm8) ^ C where C is a splatted 8-bit constant and
transforms it to vgf2p8affineqb(x, m, imm8 ^ C), eliminating the
unnecessary XOR instruction.
- The optimization runs during the combine phase after type legalization
- Handles XOR with the constant on either side (commutative)
- Only applies when the GFNI instruction has a single use to avoid
de-optimization
- Validates that the XOR operand is a splatted 8-bit constant before
folding
- Includes test coverage for positive cases and negative cases
(multi-use, non-splat constant, variable XOR)