cmd/compile: add | operator to make rewrite rules more succinct

Instead of

(And64 x x) -> x
(And32 x x) -> x
(And16 x x) -> x
(And8  x x) -> x

we can now do:

(And(64|32|16|8) x x) -> x

Any part of an opcode can have a parenthesized, |-separated list of possibilites.
The rule is then expanded using each piece of the | combo.
If there are multiple | clauses, they get expanded in tandem.
(All the first positions, then all the second positions, etc.)
All places | opcodes appear must have the same count.

A more complicated example:

(MOV(L|SS)load [off1] {sym1} (LEAQ4 [off2] {sym2} ptr idx) mem) && is32Bit(off1+off2) && canMergeSym(sym1, sym2) ->
	(MOV(L|SS)loadidx4 [off1+off2] {mergeSym(sym1,sym2)} ptr idx mem)

This meta-rule generates 2 rules, a MOVL and a MOVSS rule.

This CL is carefully orchestrated to not change the generated rules file at all.
In some cases, this means we can't align the rules nicely because it changes
the whitespace in the generated code.  I'll clean that up as a separate step.

There are many more opportunites to compactify rules using this new mechanism.
I've just done some examples, there's more to do.

Change-Id: I8a5e748cd0761ccbb12d09b01925b2f1f4b2f608
Reviewed-on: https://go-review.googlesource.com/86595


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>