[AVX] Optimize x86 VSELECT instructions using SimplifyDemandedBits.

We know that the blend instructions only use the MSB, so if the mask is
sign-extended then we can convert it into a SHL instruction. This is a
common pattern because the type-legalizer sign-extends the i1 type which
is used by the LLVM-IR for the condition.

Added a new optimization in SimplifyDemandedBits for SIGN_EXTEND_INREG -> SHL.

llvm-svn: 148225