[x86][CGP] enable target hook to sink funnel shift intrinsic's splatted shift amount

SDAG suffers when it can't see that a funnel operand is a splat value
(due to single-basic-block visibility), so invert the normal loop
hoisting rules to move a splat op closer to its use.

This would be part 1 of an enhancement similar to D63233.

This is needed to re-fix PR37426:
https://bugs.llvm.org/show_bug.cgi?id=37426
...because we got better at canonicalizing IR to funnel shift intrinsics.

The existing CGP code for shift opcodes is likely overstepping what it was
intended to do, so that will be fixed in a follow-up.

Differential Revision: https://reviews.llvm.org/D79718