[InstCombine] canonicalize cast before unary shuffle

We could go either direction on this transform. VectorCombine already goes this
way for bitcasts (and handles more complicated cases using the cost model), so
let's try cast-first.

Deferring completely to VectorCombine is another possibility. But the backend
should be able to invert this easily when the vectors have the same shape, so
it doesn't seem like a transform that we need to avoid.

The motivating example from https://llvm.org/PR49081 has an int-to-float
sandwiched between 2 shuffles, and the backend currently does not reduce that,
so on x86, we get something like:

  pshufd	$249, %xmm0, %xmm0]
  cvtdq2ps	%xmm0, %xmm0
  shufps	$144, %xmm0, %xmm0

...instead of just a single conversion instruction.

Differential Revision: https://reviews.llvm.org/D103038