Adds the new canMergeLoad function which can be used by rules to
decide whether a load can be merged into an operation. The function
ensures that the merge will not reorder the load relative to memory
operations (for example, stores) in such a way that the block can no
longer be scheduled.

This new function enables transformations such as:

MOVD 0(R1), R2
ADD  R2, R3

to:

ADD  0(R1), R3

The two-operand form of the following instructions can now read a
single memory operand:

 - ADD
 - ADDC
 - ADDW
 - MULLD
 - MULLW
 - SUB
 - SUBC
 - SUBE
 - SUBW
 - AND
 - ANDW
 - OR
 - ORW
 - XOR
 - XORW

Improves SHA3 performance by 6-8%.

Updates #15054.

Change-Id: Ibcb9122126cd1a26f2c01c0dfdbb42fe5e7b5b94
Reviewed-on: https://go-review.googlesource.com/29272
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>