[PowerPC] swdiv_nochk Builtins for XL Compat

This patch is in a series of patches to provide builtins for
compatibility with the XL compiler. This patch adds software divide
builtins with no checking. These builtins are each emitted as a fast
fdiv.

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D106150