[target/116104] Fix more rtl-checking failures in ext-dce

More enable-rtl-checking fixes for ext-dce.  Very similar to the one recently
posted, this time covering more of the shift ops.

I checked all instances of CONSTANT_P guarding [U]INTVAL and fixed all that
looked wrong.  I also created a dummy assembler/linker so that I could run the
GCC testsuite on gcn and verified that wasn't tripping any rtl-checking bugs in
ext-dce anymore.

Obviously this has also gone through x86 bootstrap and regression tested.

Pushing to the trunk.

	pr target/116104
gcc/
	* ext-dce.cc (carry_backpropagate): Change more guards of [U]INTVAL to
	test CONST_INT_P rather than CONSTANT_P, fixing rtl-checking failures.