[x86][CGP] improve sinking of splatted vector shift amount operand

Expands on the enablement of the shouldSinkOperands() TLI hook in:
D79718

The last codegen/IR test diff shows what I suspected could happen - we were
sinking all splat shift operands into a loop. But that's not what we want in
general; we only want to sink the *shift amount* operand if it is a splat.

Differential Revision: https://reviews.llvm.org/D79827