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.

llvm-svn: 72891