[X86] Custom widen 128/256-bit vXi32 uint_to_fp on avx512f targets without...

[X86] Custom widen 128/256-bit vXi32 uint_to_fp on avx512f targets without avx512vl. Similar for vXi64 sint_to_fp/uint_to_fp on avx512dq without avx512vl.

Previously we widened these through isel patterns, but that
didn't work for STRICT_ nodes. Those need to be padded with
zeroes in the upper bits which is harder to do in isel patterns.