cmd/compile: add mul by ±2ⁿ code-generation tests for arm/arm64

cmd/compile: add mul by ±2ⁿ code-generation tests for arm/arm64

This change adds code generation tests for multiplication by ±2ⁿ for
arm and arm64, in preparation for a future CL which will remove the
relevant architecture-specific SSA rules (the reduction is already
performed by rules in generic.rules added in CL 36323).

Change-Id: Iebdd5c3bb2fc632c85888569ff0c49f78569a862
Reviewed-on: https://go-review.googlesource.com/75752
Reviewed-by: Cherry Zhang <cherryyz@google.com>