PR target/47949: Use xchg to move from/to AX_REG with -Oz on x86.

This patch adds a peephole2 to i386.md to implement the suggestion in
PR target/47949, of using xchg instead of mov for moving values to/from
the %rax/%eax register, controlled by -Oz, as the xchg instruction is
one byte shorter than the move it is replacing.

The new test case is taken from the PR:
int foo(int x) { return x; }

where previously we'd generate:
foo:	mov %edi,%eax  // 2 bytes
	ret

but with this patch, using -Oz, we generate:
foo:	xchg %eax,%edi	// 1 byte
	ret

On the CSiBE benchmark, this saves a total of 10238 bytes (reducing
the -Oz total from 3661796 bytes to 3651558 bytes, a 0.28% saving).

Interestingly, some modern architectures (such as Zen 3) implement
xchg using zero latency register renaming (just like mov), so in theory
this transformation could be enabled when optimizing for speed, if
benchmarking shows the improved code density produces consistently
better performance.  However, this is architecture dependent, and
there may be interactions using xchg (instead a single_set) in the
late RTL passes (such as cprop_hardreg), so for now I've restricted
this to -Oz.

2022-08-03  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	PR target/47949
	* config/i386/i386.md (peephole2): New peephole2 to convert
	SWI48 moves to/from %rax/%eax where the src is dead to xchg,
	when optimizing for minimal size with -Oz.

gcc/testsuite/ChangeLog
	PR target/47949
	* gcc.target/i386/pr47949.c: New test case.