go/ssa: emit short-circuit control flow for boolean switch cases

Previously, switch { case x && y: ... } would materialize the
condition as a boolean. In this case, because it contains &&,
it would be a phi node. This extra phi caused the yield analyzer
to emit false positives compared to the equivalent if/else chain.

Now, the SSA builder emits short-circuit control flow for boolean
switch cases. Happily, this crosses off a TODO that has been there
since day 1: https://go.dev/issue/77681#issuecomment-3969760732A

For example, this input:

	func f(x, y bool) {
		switch {
		case x && y:
			print("hello")
		}
	}

formerly generated this SSA code:

	0:
		if x goto 3 else 4
	1:
		return
	2:
		t0 = print("hello":string)
		jump 1
	3:
		jump 4
	4:
		t1 = phi [0: false:bool, 3: y
		if t1 goto 2 else 1

but now generates this code:

	0:
		if x goto 3 else 1
	1:
		return
	2:
		t0 = print("hello":string)
		jump 1
	3:
		if y goto 2 else 1

+ regression test for gopls' yield analyzer

Fixes golang/go#77681

Change-Id: Ife9dee8443c0f2c09e95cacbfcefd5baacc97d99
Reviewed-on: https://go-review.googlesource.com/c/tools/+/749580
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>