cmd/compile: turn off SSA internal consistency checks

They've been on for a few weeks of general use and nothing
has tripped up on them yet.

Makes the compiler ~18% faster.

Change-Id: I42d7bbc0581597f9cf4fb28989847814c81b08a2
Reviewed-on: https://go-review.googlesource.com/20741Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/ssa/compile.go

@@ -117,7 +117,7 @@ type pass struct {
 }
 
 // Run consistency checker between each phase
-var checkEnabled = true
+var checkEnabled = false
 
 // PhaseOption sets the specified flag in the specified ssa phase,
 // returning empty string if this was successful or a string explaining