[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.