[PATCH v2] RISC-V: zero_extend(not) -> xor optimization [PR112398]

This patch adds optimization of the following patterns:

  (zero_extend:M (subreg:N (not:O==M (X:Q==M)))) ->
  (xor:M (zero_extend:M (subreg:N (X:M)), mask))
    ... where the mask is GET_MODE_MASK (N).

For the cases when X:M doesn't have any non-zero bits outside of mode N,
(zero_extend:M (subreg:N (X:M)) could be simplified to just (X:M)
and whole optimization will be:

  (zero_extend:M (subreg:N (not:M (X:M)))) ->
  (xor:M (X:M, mask))

Patch targets to handle code patterns like:
  not   a0,a0
  andi  a0,a0,0xff
to be optimized to:
  xori  a0,a0,255

Change was locally tested for x86_64 and AArch64 (as most common)
and for RV-64 and MIPS-32 targets (as having an effect from this optimization):
no regressions for all cases.

	PR rtl-optimization/112398
gcc/ChangeLog:

	* simplify-rtx.cc (simplify_context::simplify_unary_operation_1):
	Simplify ZERO_EXTEND (SUBREG (NOT X)) to XOR (X, GET_MODE_MASK(SUBREG))
	when X doesn't have any non-zero bits outside of SUBREG mode.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/pr112398.c: New test.

Signed-off-by: Alexey Merzlyakov <alexey.merzlyakov@samsung.com>