Eliminate more comparisons on the H8 port

This patch fixes a minor code quality issue I found while testing LRA on the
H8.  Specifically we have a peephole which converts a comparison of a memory
location against zero into a load + comparison which is actually more
efficient.  This triggers when there are registers available at the right
point during peephole2.

If the load is not a mode dependent address we can actually do better by
realizing the load itself sets the proper flags and eliminate the comparison.
I may have expected this to happen when I wrote the original peephole2,
but cmpelim runs before peephole2, so clearly if we want to eliminate the
comparison we have to do it manually.

gcc/
	* config/h8300/testcompare.md: Add peephole2 which uses a memory
	load to set flags, thus eliminating a compare against zero.

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