[X86] When commuting SSE immediate blend, make sure that the new blend mask is a valid imm8.

Example:
define <4 x i32> @test(<4 x i32> %a, <4 x i32> %b) {
  %shuffle = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32> <i32 4, i32 5, i32 6, i32 3>
  ret <4 x i32> %shuffle
}

Before llc (-mattr=+sse4.1), produced the following assembly instruction:
  pblendw $4294967103, %xmm1, %xmm0

After
  pblendw $63, %xmm1, %xmm0

llvm-svn: 221455