This fixes several test on ARM.

This fixes opt/reassoc031.c.

This gives the finish phase enough room to copy registers if
necessary.

Instead of mvn + and we get a single bic now.

The case for <= 255 just below includes 0 and gives the same result.

The spiller crashes on some unoptimized Phi nodes.

Fixes test backend/amd64_source_am.c

Now all users of be_mask_asm() can handle early clobbers automatically.

Now all users of be_make_asm() automatically use this logic.