[InstCombine] fold shuffles with FP<->Int cast operands

shuffle (cast X), (cast Y), Mask --> cast (shuffle X, Y, Mask)

This is similar to a recent transform with fneg ( b331a7eb ),
but this is intentionally the most conservative first step to
try to avoid regressions in codegen. There are several
restrictions that could be removed as follow-up enhancements.

Note that a cast with a unary shuffle is currently canonicalized
in the other direction (shuffle after cast - D103038 ). We might
want to invert that to be consistent with this patch.