[X86] Add custom promotion of narrow fp_to_uint/fp_to_sint operations under...

[X86] Add custom promotion of narrow fp_to_uint/fp_to_sint operations under -x86-experimental-vector-widening-legalization.

This tries to force the result type to vXi32 followed by a truncate. This can help avoid scalarization that would otherwise occur.

There's some annoying examples of an avx512 truncate instruction followed by a packus where we should really be able to just use one truncate. But overall this is still a net improvement.

llvm-svn: 347105