[dev.typeparams] go/types: clean up index expr implementation for type parameters

[dev.typeparams] go/types: clean up index expr implementation for type parameters

This is a port of CL 332553 to go/types. The "expr" variable is renamed to
"e" in Checker.indexExpr to be consistent with types2.

Change-Id: I7905bebf2e8dab47256361362b16becf7596cf95
Reviewed-on: https://go-review.googlesource.com/c/go/+/335110
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Robert Findley <rfindley@google.com>