[dev.ssa] cmd/compile: clean up nilcheck logic

[dev.ssa] cmd/compile: clean up nilcheck logic

Be more clear about the two conditions that we care about:
1) a block that performs a nil check (OpIsNonNil), which may be removed
2) a block that is the non-nil sucessor for an OpIsNonNil block

Now we only care about removing nilchecks for two scenarios:
- a type 1 block is dominated by a type 2 block for the same value
- a block is both type 1 and type 2 for the same value

Fixes math/big.

Change-Id: I50018a4014830461ddfe2a2daf588468e4a8f0b4
Reviewed-on: https://go-review.googlesource.com/14325
Reviewed-by: Keith Randall <khr@golang.org>