[X86] Use default expansion for (i64 fp_to_uint f80) when avx512 is enabled on...

[X86] Use default expansion for (i64 fp_to_uint f80) when avx512 is enabled on 64-bit targets to match what happens without avx512.

In 64-bit mode prior to avx512 we use Expand, but with avx512 we need to make f32/f64 conversions Legal so we use Custom and then do our own expansion for f80. But this seems to produce codegen differences relative to avx2. This patch corrects this.

llvm-svn: 353921