cmd/compile: cannot elim binary search typeHash on a type switch for a known type

[wdvxdr1123]

In cmd/compile/internal/walk/switch.go, gc rewrites type switch into a binary search for typeHash and some type assertions.
The SSA backend can correctly eliminate the type assertions for a known type, but can't eliminate the binary search.

What did you do?

package a

func test(y int) int {
	var x = interface{}(y)
	var a = 1
	switch x.(type) {
	case int:
		a = 2
	case string:
		a = 3
	case int32:
		a = 4
	default:
		a = 5
	}
	return a
}

What did you expect to see?

00000 (3) TEXT "".test(SB), ABIInternal
    MOVL $2, AX
    RET

What did you see instead?

00000 (3) TEXT "".test(SB), ABIInternal
00001 (3) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
00002 (3) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
00003 (3) FUNCDATA $5, "".test.arginfo1(SB)
00004 (+6) MOVL type.int+16(SB), CX
00005 (6) CMPL CX, $-1146273534
00006 (6) JEQ 13
00007 (6) CMPL CX, $-520135500
00008 (6) JEQ 13
00009 (6) CMPL CX, $-145526278
00010 (6) JNE 13
00011 (+6) MOVL $2, AX
00012 (6) JMP 14
00013 (6) MOVL $5, AX
00014 (+16) RET
00015 (?) END