[X86] Simplify the setOperationAction handling for fp_to_uint by improving the...

[X86] Simplify the setOperationAction handling for fp_to_uint by improving the Custom handler a bit.

This merges the 32-bit and 64-bit mode code to just use Custom
for both i32 and i64. We already had most of the handling in
the custom handling due to the AVX512 having legal fp_to_uint.
Just needed to add the i32->i64 promotion handling. Refactor
the fp_to_uint code in the custom handler to simplify the
number of times we check things.

Tweak cost model tables to match the default handling we were
getting due to Expand before.

llvm-svn: 370700