JIT: Fix illegal instruction encoding for float->float casts

[saucecontrol]

Fixes #112163

This resolves the illegal instruction encoding by no longer pretending that movss and movsd are 'conversion' instructions, which have different encoding forms.

There's a minor codegen improvement (diffs) when a no-op cast has different source and target registers, where we now emit movaps instead of movss/movsd. Previously we would elide the cast only if source and target reg were the same.

-       vmovss   xmm1, xmm1, xmm2
+       vmovaps  xmm1, xmm2
        vbroadcastss xmm1, xmm1

[EgorBo]

/azp run runtime-coreclr jitstress-isas-avx512

[azure-pipelines]

Azure Pipelines could not run because the pipeline triggers exclude this branch/path.

[EgorBo]

/azp list