[PowerPC] Fix signature of lxvp and stxvp builtins

This patch changes the signature of the load and store vector pair
builtins to match their documentation. The type of the `signed long long`
argument is changed to `signed long`. This patch also changes existing testcases
to match the signature change.

Reviewed By: lei, Conanap

Differential Revision: https://reviews.llvm.org/D109996