[PowerPC][NFC] Rename P10 builtins vec_clrl, vec_clrr to vec_clr_first and vec_clr_last

This patch renames the vector clear left/right builtins vec_clrl, vec_clrr to
vec_clr_first and vec_clr_last to avoid the ambiguities when dealing with endianness.

Reviewed By: amyk, lei

Differential revision: https://reviews.llvm.org/D108702