[PowerPC] Add range checks for P10 Vector Builtins

This patch adds range checking for some Power10 altivec builtins and
changes the signature of a builtin to match documentation. For `vec_cntm`,
range checking is done via SemaChecking. For `vec_splati_ins`, the second
argument is masked to extract the 0th bit so that we always receive either a `0`
or a `1`.

Reviewed By: lei, amyk

Differential Revision: https://reviews.llvm.org/D109710