[dev.regabi] cmd/compile: minor walkExpr cleanups

This CL cleans up a few minor points in walkExpr:

1. We don't actually care about computing the type-size of all
expressions that are walked. We care about computing the type-size of
all expressions that are *returned* by walk, as these are the
expressions that will actually be seen by the back end.

2. There's no need to call typecheck.EvalConst anymore. EvalConst used
to be responsible for doing additional constant folding during walk;
but for a while a now, it has done only as much constant folding as is
required during type checking (because doing further constant folding
led to too many issues with Go spec compliance). Instead, more
aggressive constant folding is handled entirely by SSA.

3. The code for detecting string constants and generating their
symbols can be simplified somewhat.

Passes toolstash -cmp.

Change-Id: I464ef5bceb8a97689c8f55435369a3402a5ebc55
Reviewed-on: https://go-review.googlesource.com/c/go/+/280434


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>