cmd/compile: add OpAMD64CMPptr and friends?

[josharian]

amd64.rules contains a fair number of rules that are conditional on config.PtrSize, to support amd64p32. See CL 46870 for a recent example. To reduce duplication, I suggest we add ops whose lowering depends on pointer size. So for example, instead of:

(IsInBounds idx len) && config.PtrSize == 8 -> (SETB (CMPQ idx len))
(IsInBounds idx len) && config.PtrSize == 4 -> (SETB (CMPL idx len))

we would have only:

(IsInBounds idx len) -> (SETB (CMPptr idx len))

where CMPptr is treated as CMPQ for amd64 and CMPL for amd64p32. Similarly so for other such duplicated rules.

Opinions or objections, @randall77 @cherrymui?

[randall77]

Sounds ok to me. You are, however, trading verbosity in some places for verbosity elsewhere.
You're removing a IsInBounds rule, but then you need a CMPconstptr op, rules to generate that op, etc.
It's not clear to me that it is a net win. If it is, go for it.

[dr2chase]

No recent progress, seems like this is not setting anyone's hair on fire, moving to unplanned.