[ARM][atomicrmw] Fix CMP_SWAP_32 expand assert

This assert is intended to ensure that the high registers are not
selected when it is passed to one of the thumb UXT instructions. However
it was triggering even for 32 bit where no UXT instruction is emitted.

Fixes PR51313.

Differential Revision: https://reviews.llvm.org/D107363

(cherry picked from commit 40650f27)