[PowerPC] Implement vector bool/pixel initialization under -faltivec-src-compat=xl

This patch implements the initialization of vectors under the
-faltivec-src-compat=xl option introduced in https://reviews.llvm.org/D103615.

Under this option, the initialization of scalar vectors, vector bool, and vector
pixel are treated the same, where the initialization value is splatted across
the whole vector.

This patch does not change the behaviour of the -faltivec-src-compat=mixed option,
which is the current default for Clang.

Differential Revision: https://reviews.llvm.org/D106120