[PowerPC] Expand constrained ppc_fp128 to i32 conversion

Libcall __gcc_qtou is not available, which breaks some tests needing
it. On PowerPC, we have code to manually expand the operation, this
patch applies it to constrained conversion. To keep it strict-safe,
it's using the algorithm similar to expandFP_TO_UINT.

For constrained operations marking FP exception behavior as 'ignore',
we should set the NoFPExcept flag. However, in some custom lowering
the flag is missed. This should be fixed by future patches.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D86605