Replaced comparison based on (*Type).String() with an
allocation-free structural comparison.  Roughly doubles
speed of CSE, also reduces allocations.

Checked that roughly the same number of CSEs were detected
during make.bash (about a million) and that "new" CSEs
were caused by the effect described above.

Change-Id: Id205a9f6986efd518043e12d651f0b01206aeb1b
Reviewed-on: https://go-review.googlesource.com/19471Reviewed-by: Keith Randall <khr@golang.org>