When a constant doesn't fit in a single instruction, use two
paired instructions instead of the constant pool.  For example

  ADD $0xaa00bb, R0, R1

Used to rewrite to:

  MOV ?(IP), R11
  ADD R11, R0, R1

Instead, do:

  ADD $0xaa0000, R0, R1
  ADD $0xbb, R1, R1

Same number of instructions.
Good:
  4 less bytes (no constant pool entry)
  One less load.
Bad:
  Critical path is one instruction longer.

It's probably worth it to avoid the loads, they are expensive.

Dave Cheney got us some performance numbers: https://perf.golang.org/search?q=upload:20170426.1
TL;DR mean 1.37% improvement.

Change-Id: Ib206836161fdc94a3962db6f9caa635c87d57cf1
Reviewed-on: https://go-review.googlesource.com/41612
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>