Changed racewalk/race detector to use FP in a more
sensible way.

Relaxed checks for CONVNOP when race detecting.

Modified tighten to ensure that GetClosurePtr cannot float
out of entry block (turns out this cannot be relaxed, DX is
sometimes stomped by other code accompanying race detection).

Added case for addr(CONVNOP)

Modified addr to take "bounded" flag to suppress nilchecks
where it is set (usually, by race detector).

Cannot leave unimplemented-complainer enabled because it
turns out we are optimistically running SSA on every platform.

Change-Id: Ife021654ee4065b3ffac62326d09b4b317b9f2e0
Reviewed-on: https://go-review.googlesource.com/15710Reviewed-by: Keith Randall <khr@golang.org>