[dev.ssa] cmd/compile: implement the following for SSA on ARM

[dev.ssa] cmd/compile: implement the following for SSA on ARM

- generic Ops: Phi, CALL variants, NilCheck
- generic Blocks: Plain, Check
- 32-bit arithmetics
- CMP and conditional branches
- load/store
- zero/sign-extensions (8 to 16, 8 to 32, 16 to 32)

Progress on SSA backend for ARM. Still not complete. Now "errors"
package compiles and tests passed.

Updates #15365.

Change-Id: If126fd17f8695cbf55d64085bb3f1a4a53205701
Reviewed-on: https://go-review.googlesource.com/22856
Reviewed-by: Keith Randall <khr@golang.org>