cmd/compile: get untyped constants working in generic functions

types2 will give us a constant with a type T, if an untyped constant is
used with another operand of type T (in a provably correct way). When we
substitute in the type args during stenciling, we now know the real type
of the constant. We may then need to change the BasicLit.val to be the
correct type (e.g. convert an int64Val constant to a floatVal constant).
Otherwise, later parts of the compiler will be confused.

Updated tests list.go and double.go with uses of untyped constants.

Change-Id: I9966bbb0dea3a7de1c5a6420f8ad8af9ca84a33e
Reviewed-on: https://go-review.googlesource.com/c/go/+/303089


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>