[AArch64][GlobalISel] Fix one use check in getTestBitReg

(1) The check needs to be on the 0th operand of whatever we're folding
(2) Checks for validity should happen before we change the bit

Fixes a bug which caused MultiSource/Applications/JM/lencod to fail at -O3.

Differential Revision: https://reviews.llvm.org/D74002