JIT: Move remaining xarch floating->integral cast implementation to lowering

[saucecontrol]

This changes the lowering of floating->integral casts to always replace the cast node with HWIntrinsics rather than doing fixups ahead of the cast and leaving the node in place as a self-cast or letting it be handled in codegen. Since the self-cast was not always eliminated in codegen, this results in some size and throughput improvements.

Because the cast is always replaced now, genFloatToIntCast is no longer necessary on xarch.

This is best viewed with whitespace ignored. Most of the changes are simply an extra level of indentation for the pre-AVX10.2 code in LowerCast.

Diffs

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[saucecontrol]

Looks like this change was missed in #111861

Since floating->integral casts rely on at least baseline intrinsics support, FEATURE_HW_INTRINSICS is no longer optional on x86. Seems linux-x86 has been building because the implementation was #ifdef`ed, but it would have failed at run time.

[saucecontrol]

This intrinsic doesn't support float args, so it was misplaced in the previous grouping.

[saucecontrol]

cc @tannergooding

This is more peeled from #116805, with feedback addressed

[tannergooding]

CC. @dotnet/jit-contrib, @EgorBo for secondary sign-off

This has a couple correctness fixes in addition to the general codegen improvements, so if we're not comfortable taking the whole thing for .NET 10, we likely still need to peel off the fixes that were called out