Add new constant-flags opcodes.  These can be generated from
comparisons that we know the result of, like x&31 < 32.

Constant-fold the constant-flags opcodes into all flag users.

Reorder some CMPxconst args so they read in the comparison direction.

Reorg deadcode removal a bit - it needs to remove the OpCopy ops it
generates when strength-reducing Phi ops.  So it needs to splice out all
the dead blocks and do a copy elimination before it computes live
values.

Change-Id: Ie922602033592ad8212efe4345394973d3b94d9f
Reviewed-on: https://go-review.googlesource.com/18267
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>