cmd/compile: optimize escape graph construction and walking

This CL implements several optimizations for the escape analysis flow
graph:

1. Instead of recognizing heapLoc specially within Escape.outlives,
set heapLoc.escapes = true and recognize any location with escapes
set. This allows us to skip adding edges from the heap to escaped
variables in two cases:

1a. In newLoc, if the location is for a variable or allocation too
large to fit on the stack.

1b. During walkOne, if we discover that an object's address flows
somewhere that naturally outlives it.

2. When recording edges in Escape.flow, if x escapes and we're adding
an edge like "x = &y", we can simply mark that y escapes too.

3. During walkOne, if we reach a location that's marked as escaping,
we can skip visiting it again: we've either already walked from it, or
it's in queue to be walked from again.

On average, reduces the number of visited locations by 15%. Reduces
time spent in escape analysis for particularly hairy packages like
runtime and gc by about 8%. Reduces escape.go's TODO count by 22%.

Passes toolstash-check.

Change-Id: Iaf86a29d76044e4b4c8ab581b916ef5bb5df4437
Reviewed-on: https://go-review.googlesource.com/c/go/+/196811


Reviewed-by: Cherry Zhang <cherryyz@google.com>