This adds rules to match the code in math/bits RotateLeft,
RotateLeft32, and RotateLef64 to allow them to be inlined.

The rules are complicated because the code in these function
use different types, and the non-const version of these
shifts generate Mask and Carry instructions that become
subexpressions during the match process.

Also adds a testcase to asm_test.go.

Improvement in math/bits:

BenchmarkRotateLeft-16       1.57     1.32      -15.92%
BenchmarkRotateLeft32-16     1.60     1.37      -14.37%
BenchmarkRotateLeft64-16     1.57     1.32      -15.92%

Updates #21390

Change-Id: Ib6f17669ecc9cab54f18d690be27e2225ca654a4
Reviewed-on: https://go-review.googlesource.com/59932
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>