AMDGPU: Clear kill flags after FoldZeroHighBits (#99582)
After folding, all uses of the result register are going to be replaced
by the operand register. The kill flags on the uses of the result and
operand registers are no longer valid after the replacement, and need to
be cleared.
The only exception is, however, if the kill flag is set for the operand
register, we are sure the last use of the result register is the new
last use of the operand register, and thus we are safe to keep the kill
flags.
