[NFC] [PowerPC] Remove dead code in BUILD_VECTOR peephole

The piece of code tries to use splat+shift to lower build_vector with
repeating bit pattern. And immediate field of vector splat is only 5
bits (-16~15). It iterates over them one by one to find which
shifts/rotates to number in build_vector.

This patch removes code to try matching constant with algebraic
right-shift because that's meaningless - any negative number's algebraic
right-shift won't produce result smaller than itself. Besides, code
(int)((unsigned)i >> j) means logical shift-right in C.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D93937