simplify-rtx: Add `(subreg (not a))` simplification for word_mode [PR121308]

simplify-rtx: Add `(subreg (not a))` simplification for word_mode [PR121308]

Right now in simplify_subreg, there is code to try to simplify for word_mode
with the binary bitwise operators. The unary bitwise operator is not handle,
this causes an odd mix match and the new self testing code that was added with
r16-2614-g965564eafb721f was not expecting.

The self testing code was for testing the newly added code but since there
was already code that handles word_mode, we hit the mismatch but only
for targets where word_mode is SImode (or smaller).

This adds the code to handle `not` in a similar fashion as the other
bitwise operators for word_mode.

Changes since v1:
* v2: add `&& SCALAR_INT_MODE_P (innermode)` to the conditional.

Bootstrapped and tested on x86_64-linux-gnu.

	PR rtl-optimization/121308
gcc/ChangeLog:

	* simplify-rtx.cc (simplify_context::simplify_subreg): Handle
	subreg of `not` with word_mode to make it symmetric with the
	other bitwise operators.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>