[PowerPC] Stash GPR to VSR if emergency spill slot is not reachable

[PowerPC] Stash GPR to VSR if emergency spill slot is not reachable

When removing frame indices on PowerPC, we need to scavenge
a GPR to materialize a large constant if the stack offset
for the spill/reload cannot be reached by a D-Form
instruction. However, in a perfect storm of conditions,
we may not have GPR's available to scavenge, thereby
requiring an emergency spill. If such an emergency
spill also needs to be spilled to a location with a
large offset, it would itself require register scavenging
thereby creating an infinite loop.

This patch detects when the scavenger cannot scavenge
a register and the spill/reload is to a location with
a large offset. It then stashes a GPR into a VSR so
that it can use the GPR to materialize the constant
(rather than scavenging a GPR).

Fixes: https://github.com/llvm/llvm-project/issues/52894
Differential revision: https://reviews.llvm.org/D124841