[dev.typeparams] cmd/compile: fix small -G=3 issues for tests disabled in run.go

 - set correct position for closure capture variable in (*irgen).use()
   (issue20250.go) Also, evaluate rhs, lhs in that order in assignment
   statements to match noder1 (affects ordering of closure variables).

 - make sure to set Assign flag properly in (*irgen).forStmt() for range
   variables which are map accesses (issue9691.go)

 - make sure CheckSize() is call on the base type for top-level types
   converted by (*irgen).typ() that are pointer types (issue20174.go and
   issue37837.go)

 - deal with parentheses properly in validation function
   (*irgen).validate() (issue17270.go)

 - avoid HasNil call on type TTYPEPARAM - types2 typechecker will have
   already checked validity of the typeparam having nil value (new test
   issue39755.go)

Change-Id: Ie68004d964698aea047e19e7dcd79b297e9d47ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/334733


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>