cmd/compile: sometimes issue rematerializable values early

[josharian]

package p

var (
	s *int
	b bool
)

func f() {
	var q *int
	if b {
		q = new(int)
	} else {
		q = new(int)
	}
	s = q
}

This code is a bit silly, but it's the smallest reproduction I have handy. :)

When compiled, this code tests b, and on each branch it contains an LEAQ of type.int. The LEAQ should be hoisted above the branch, since it is needed immediately in each case, and there are registers to spare.

The CSE pass does its work, and there is a single LEAQ instruction going into regalloc. However, early in regalloc, we decide to issue it every place we need it:

			if s.values[v.ID].rematerializeable {
				// Value is rematerializeable, don't issue it here.
				// It will get issued just before each use (see
				// allocValueToReg).
				for _, a := range v.Args {
					a.Uses--
				}
				s.advanceUses(v)
				continue
			}

This is usually the right decision--disabling this bit of code causes an overall regression. But as the initial code shows, there are instances in which it'd be better to issue the rematerializable value right away. I'm not sure exactly what the right set of conditions is, though.

cc @cherrymui @randall77