cmd/compile: repair MININT conversion bug in arm softfloat

Negative-case conversion code was wrong for minimum int32,
used negate-then-widen instead of widen-then-negate.

Test already exists; this fixes the failure.

Fixes #15563.

Change-Id: I4b0b3ae8f2c9714bdcc405d4d0b1502ccfba2b40
Reviewed-on: https://go-review.googlesource.com/22830
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/softfloat_arm.go

@@ -530,7 +530,7 @@ execute:
 	case 0xeeb80ac0: // D[regd] = S[regm] (MOVWF)
 		cmp := int32(m.freglo[regm])
 		if cmp < 0 {
-			fputf(regd, f64to32(fintto64(int64(-cmp))))
+			fputf(regd, f64to32(fintto64(-int64(cmp))))
 			m.freglo[regd] ^= 0x80000000
 		} else {
 			fputf(regd, f64to32(fintto64(int64(cmp))))
@@ -552,7 +552,7 @@ execute:
 	case 0xeeb80bc0: // D[regd] = S[regm] (MOVWD)
 		cmp := int32(m.freglo[regm])
 		if cmp < 0 {
-			fputd(regd, fintto64(int64(-cmp)))
+			fputd(regd, fintto64(-int64(cmp)))
 			m.freghi[regd] ^= 0x80000000
 		} else {
 			fputd(regd, fintto64(int64(cmp)))