PR tree-optimization/98335: New peephole2 xorl;movb -> movzbl

PR tree-optimization/98335: New peephole2 xorl;movb -> movzbl

This patch is the backend piece of my proposed fix to PR tree-opt/98335,
to allow C++ partial struct initialization to be as efficient/optimized
as full struct initialization.

With the middle-end patch just posted to gcc-patches, the test case
in the PR compiles on x86_64-pc-linux-gnu with -O2 to:

        xorl    %eax, %eax
        movb    c(%rip), %al
        ret

with this additional peephole2 (actually four peephole2s):

        movzbl  c(%rip), %eax
        ret

2022-03-11  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR tree-optimization/98335
	* config/i386/i386.md (peephole2): Eliminate redundant insv.
	Combine movl followed by movb.  Transform xorl followed by
	a suitable movb or movw into the equivalent movz[bw]l.

gcc/testsuite/ChangeLog
	PR tree-optimization/98335
	* g++.target/i386/pr98335.C: New test case.
	* gcc.target/i386/pr98335.c: New test case.