Add & as an input op.  Add several output ops (loads & stores, TESTB,
LEAQglobal, branches, memcopy)

Some other small things:
- Add exprAddr to builder to generate addresses of expressions.  Use it in
  various places that had ad-hoc code.
- Separate out nil & bounds check generation to separate functions.
- Add explicit FP and SP ops so we dont need specialized *FP and *SP opcodes.
- Fix fallthrough at end of functions with no return values.
- rematerialization of more opcodes.

Change-Id: I781decfcef9770fb15f0cd6b061547f7824a2d5e
Reviewed-on: https://go-review.googlesource.com/10213Reviewed-by: Alan Donovan <adonovan@google.com>