Fix FP_TO_UINT->i32 on ppc32 -mcpu=g5. This was

Fix FP_TO_UINT->i32 on ppc32 -mcpu=g5.  This was
using Promote which won't work because i64 isn't
a legal type.  It's easy enough to use Custom, but
then we have the problem that when the type
legalizer is promoting FP_TO_UINT->i16, it has no
way of telling it should prefer FP_TO_SINT->i32
to FP_TO_UINT->i32.  I have uncomfortably hacked
this by making the type legalizer choose FP_TO_SINT
when both are Custom.
This fixes several regressions in the testsuite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72891 91177308-0d34-0410-b5e6-96231b3b80d8