cmd/compile: store combine not working on ppc64 and arm64 with global variables

[rasky]

While migrating the code generation tests for load/store combiners to to the top-level testsuite (fad31e5), I had to disable some tests for arm64 and ppc64le because it looks like that store combining is failing in some situations.

For instance:

go/test/codegen/memcombine.go

Lines 132 to 137 in fad31e5

	func store_le16_idx(b []byte, idx int) {
	// amd64:`MOVW\s`
	// arm64(DISABLED):`MOVH`,-`MOVB`
	// ppc64le(DISABLED):`MOVH\s`
	binary.LittleEndian.PutUint16(b[idx:], sink16)
	}

It looks like the bug is related to the fact that the store is made from a global variable rather than a local variable (as was being previously tested).

[rasky]

/cc @laboger @williamweixiao @mrosier-qdt

[gopherbot]

Change https://golang.org/cl/98397 mentions this issue: cmd/compile/internal/ssa: improve store combine optimization on arm64

[laboger]

I noticed this regression in combining little endian stores while testing my current change for combining loads and stores in big endian order on ppc64le. I have a fix that I was going to include with the big endian change, but could make it separate if that is preferred.

[gopherbot]

Change https://golang.org/cl/98136 mentions this issue: cmd/compile,test: combine byte loads and stores on ppc64le