x86_64: Use peephole2 to eliminate redundant moves.

The recent fix for mul_widen_cost revealed an interesting
quirk of ira/reload register allocation on x86_64.  As shown in
https://gcc.gnu.org/pipermail/gcc-patches/2020-August/551648.html
for gcc.target/i386/pr71321.c we generate the following code that
performs unnecessary register shuffling.

        movl    $-51, %edx
        movl    %edx, %eax
        mulb    %dil

Various discussions in bugzilla seem to point to reload preferring
not to load constants directly into CLASS_LIKELY_SPILLED_P registers.
Whatever the cause, one solution (workaround), that doesn't involve
rewriting a register allocator, is to use peephole2 to spot this
wierdness and eliminate it.  With this peephole2 the above three
instructions (from pr71321.c) are replaced with:

        movl    $-51, %eax
        mulb    %dil

2020-08-12  Roger Sayle  <roger@nextmovesoftware.com>
	    Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (peephole2): Reduce unnecessary
	register shuffling produced by register allocation.

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