[dev.typeparams] cmd/compile: more incremental typecheck for unified IR

CL 332469 changed the unified IR reader to incrementally typecheck
each statement as they're read/constructed. This CL goes further to
incrementally typecheck each expression.

While here, this CL reorganizes a few things to make this go more
smoothly. In particular, it renames expr to expr0 and adds a new expr
wrapper that applies typecheck.Expr; gets rid of exprTypeSwitchguard
by moving that logic into switchStmt; and splits exprConvert out from
exprCall, which simplifies the logic for typechecking the calleee
expression somewhat.

Change-Id: I6289de9388dc94a947971f4b7213aafeb2faa5dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/333730


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