[PowerPC] Implemented Vector Load with Zero and Signed Extend Builtins

This patch implements the builtins for Vector Load with Zero and Signed Extend Builtins (lxvr_x for b, h, w, d), and adds the appropriate test cases for these builtins. The builtins utilize the vector load instructions itnroduced with ISA 3.1.

Differential Revision: 	https://reviews.llvm.org/D82502#inline-797941