[PowerPC] Don't crash on PPC32 i64 fp_to_uint on modern cores

[PowerPC] Don't crash on PPC32 i64 fp_to_uint on modern cores

When we have an instruction for this (and, thus, don't generate a runtime
call), we need to custom type legalize this (in a trivial way, just as we do
for fp_to_sint).

Fixes PR23173.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@234561 91177308-0d34-0410-b5e6-96231b3b80d8