[PR rtl-optimization/122536] Fix guard against variable bit extracts in RTL simplifier

[PR rtl-optimization/122536] Fix guard against variable bit extracts in RTL simplifier

While I haven't been able to reproduce the failure Sam reported, based on the
RTL checking failure point and the code in question it was pretty clear the
code that was supposed to guard against a variable bit extract was just
checking the wrong argument to verify it was a constant integer -- it missed a
level of XEXP object extraction.

I passed this patch along to Sam and he verified it fixes his kernel build.
I've bootstrapped and regression tested on x86_64.  Pushing to the trunk.

Pushing to the trunk.

	PR rtl-optimization/122536
gcc/
	* simplify-rtx.cc (simplify_context::simplify_binary_operation_1): Fix
	guard against variable bit extracts in recent change.