[PowerPC] Avoid unnecessary fadd for unsigned to ppcf128

Unsigned 32-bit or shorter integer to ppcf128 conversion are currently
expanded as signed-to-double with an extra fadd to 'complement'. But on
PowerPC we have native instruction to directly convert unsigned to
double since ISA v2.06. This patch exploits it.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D89786