[X86][SSE] Combine UNPCKL with vector_shuffle into UNPCKH to save one...

[X86][SSE] Combine UNPCKL with vector_shuffle into UNPCKH to save one instruction for sext from v16i8 to v16i16 and v8i16 to v8i32.

This patch is enabling combining UNPCKL with vector_shuffle that moves the upper
half of a vector into the lower half, into a UNPCKH instruction. For example:

t2: v16i8 = vector_shuffle<8,9,10,11,12,13,14,15,u,u,u,u,u,u,u,u> t1, undef:v16i8
t3: v16i8 = X86ISD::UNPCKL undef:v16i8, t2

will be combined to:

t3: v16i8 = X86ISD::UNPCKH undef:v16i8, t1


Differential revision: http://reviews.llvm.org/D14399

llvm-svn: 253067