This patch implements custom codegen for the vec_replace_elt and vec_replace_unaligned builtins.
These builtins map to the @llvm.ppc.altivec.vinsw and @llvm.ppc.altivec.vinsd intrinsics depending on the arguments.
The main motivation for doing custom codegen for these intrinsics is because there are float and double versions of the
builtin. Normally, the converting the float to an integer would be done via fptoui in the IR. This is incorrect as fptoui
truncates the value and we must ensure the value is not truncated. Therefore, we provide custom codegen to utilize
bitcast instead as bitcasts do not truncate.
The original patch that implemented the front end done this adding unions to altivec.h (https://reviews.llvm.org/D82359) but
this patch uses custom codegen to use bitcast instead for the float conversion instead.
Depends on D83497.