[dev.ssa] cmd/compile: implement shifts & multiplications for SSA on ARM

[dev.ssa] cmd/compile: implement shifts & multiplications for SSA on ARM

Implement shifts and multiplications for up to 32-bit values.

Also handle Exit block.

Progress on SSA backend for ARM. Still not complete.
container/heap, crypto/subtle, hash/adler32 packages compile and
tests passed.

Updates #15365.

Change-Id: I6bee4d5b0051e51d5de97e8a1938c4b87a36cbf8
Reviewed-on: https://go-review.googlesource.com/23096
Reviewed-by: Keith Randall <khr@golang.org>