x86: Avoid generating orb $0, %ah

x86: Avoid generating orb $0, %ah

I'll post my proposed fix for PR target/103611 shortly, but this patch
fixes another missed optimization opportunity revealed by that PR.
Occasionally, reload materializes integer constants during register
allocation sometimes resulting in unnecessary instructions such as:

(insn 23 31 24 2 (parallel [
            (set (reg:SI 0 ax [99])
                (ior:SI (reg:SI 0 ax [99])
                    (const_int 0 [0])))
            (clobber (reg:CC 17 flags))
        ]) "pr103611.c":18:73 550 {*iorsi_1}
     (nil))

These then get "optimized" during the split2 pass, which realizes that
no bits outside of 0xff00 are set, so this operation can be implemented
by operating on just the highpart of a QIreg_operand, i.e. %ah, %bh, %ch
etc., which leads to the useless "orb $0, %ah" seen in the reported PR.

This fix catches the case of const0_rtx in relevant splitter, either
eliminating the instruction or turning it into a simple move.

2021-12-13  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (define_split any_or:SWI248 -> orb %?h):
	Optimize the case where the integer constant operand is zero.

gcc/testsuite/ChangeLog
	* gcc.target/i386/pr103611-1.c: New test case.