[Altivec] Use signed comparison for vec_all_* and vec_any_* interfaces

We are currently being inconsistent in using signed vs unsigned comparisons for
vec_all_* and vec_any_* interfaces that use vector bool types. For example we
use signed comparison for vec_all_ge(vector signed char, vector bool char) but
unsigned comparison for when the arguments are swapped. GCC and XL use signed
comparison instead. This patch makes clang consistent with itself and with XL
and GCC.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D105666