Expression::bounds_ is used only by a subset of compile passes, but the
data structure occupies space for every Expression node ever parsed. This
unneccessarily increases memory consumption. Particularly, peak memory
consumption during startup, which may cause out-of-memory errors.

This CL
- removes Expression::bounds_;
- introduces an AstTypeBounds container, which mappes Expression* to Bounds;
- modifies the code that actually requires bounds information, namely
  Crankshaft compile and AsmWasmBuilder, to instantiate such an AstTypeBounds
  container before typing and to pass it to the code that consumes this
  information; and
- modifies all accesses to Expression::bounds_ to instead access the bounds
  via the container instead.

Additionally, this rewrites test-ast-expression-visitor. The reason is that
this code attempted to test AstExpressionVisitor but did so exclusively
through its subclass ExpressionTypeCollector, meaning that the test dealt
almost exclusively with type bounds despite the class-under-test having
no knowledge or functionality related to it. Worse, the test was written
in a way to assume that type bounds were available outside & after
compilation, which is something this change changes.

BUG=v8:4947

Review-Url: https://codereview.chromium.org/1968383002
Cr-Commit-Position: refs/heads/master@{#36222}