[PowerPC] Fixing implicit castings in altivec for -fno-lax-vector-conversions

XL considers different vector types to be incompatible with each other.
For example assignment between variables of types vector float and vector
long long or even vector signed int and vector unsigned int are diagnosed.
clang, however does not diagnose such cases and does a simple bitcast between
the two types. This could easily result in program errors. This patch is to
fix the implicit casts in altivec.h so that there is no incompatible vector
type errors whit -fno-lax-vector-conversions, this is the prerequisite patch
to switch the default to -fno-lax-vector-conversions later.

Reviewed By: nemanjai, amyk

Differential Revision: https://reviews.llvm.org/D124093