cmd/compile/internal: support new(expr)

cmd/compile/internal: support new(expr)

This CL adds compiler support for new(expr),
a feature of go1.26 that allows the user to specify
the initial value of the variable instead of its
type.

Also, a basic test of dynamic behavior.

See CL 704737 for spec change and CL 704935 for
type-checker changes.

For #45624

Change-Id: I65d27de1ee3aabb819b57cce8ea77f3073447757
Reviewed-on: https://go-review.googlesource.com/c/go/+/705157
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>