operand: I32 not accepted as imm32 argument

[mmcloughlin]

From @vsivsi in https://gophers.slack.com/archives/C6WDZJ70S/p1618277888005900:

Quick Avo question. Why does this not work? Bug or feature?

tmp := GP32()
MOVL(I32(-1), tmp)   // MOVL: bad operands

What’s the point of I32() if it can’t be used to put a signed value into an imm32 ?

This is a bug, likely due to:

avo/operand/checks.go

Lines 48 to 52 in 3622eb0

	// IsIMM32 returns true is op is a 32-bit immediate.
	func IsIMM32(op Op) bool {
	_, ok := op.(U32)
	return ok
	}