Subzero, MIPS32: Fix conditional mov instructions

This patch implements changes needed for conditional mov instructions
to fix problem with failing crosstest and invalid register allocation.
Problem is visible from icmp test examples, causing cross test for icmp
to fail. Eg:

Incorrect, before this change:
674:	00653026	xor	a2,v1,a1
678:	00a3182b	sltu	v1,a1,v1
67c:	0082102b	sltu	v0,a0,v0
680:	0043180a	movz	v1,v0,v0

Correct, aftrer this change:
674:	00653026	xor	a2,v1,a1
678:	00a3182b	sltu	v1,a1,v1
67c:	0082102b	sltu	v0,a0,v0
680:	0046180a	movz	v1,v0,a2

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2394773004 .

Patch from Stefan Maksimovic <makdstefan@gmail.com>.