cmd/compile: refactor constant rewriting

cmd/compile: refactor constant rewriting

Extract all rewrite-to-OLITERAL expressions to use a single setconst
helper function.

Does not pass toolstash-check for two reasons:

1) We now consistently clear Left/Right/etc when rewriting Nodes into
OLITERALs, which results in their inlining complexity being correctly
computed. So more functions can now be inlined.

2) We preserve Pos, so PC line tables change somewhat.

Change-Id: I2b5c293bee7c69c2ccd704677f5aba4ec40e3155
Reviewed-on: https://go-review.googlesource.com/103860
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>