[dev.typeparams] cmd/compile: force untyped constants from types2 to expected kind

Currently, types2 sometimes produces constant.Values with a Kind
different than the untyped constant type's Is{Integer,Float,Complex}
info, which irgen expects to always match.

While we mull how best to proceed in #43891, this CL adapts irgen to
types2's current behavior. In particular, fixedbugs/issue11945.go now
passes with -G=3.

Updates #43891.

Change-Id: I24823a32ff49af6045a032d3903dbb55cbec6bef
Reviewed-on: https://go-review.googlesource.com/c/go/+/286652


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>