[X86] Teach the backend how to fold SSE4.1/AVX/AVX2 blend intrinsics.

Added target specific combine rules to fold blend intrinsics according
to the following rules:
 1) fold(blend A, A, Mask) -> A;
 2) fold(blend A, B, <allZeros>) -> A;
 3) fold(blend A, B, <allOnes>) -> B.

Added two new tests to verify that the new folding rules work for all
the optimized blend intrinsics.