cmd/compile: upgrade ssa to do (int or float) -> complex

Generic instantiations can produce conversions from constant
literal ints or floats to complex values. We could constant literals
during instantiation, but it is just as easy to upgrade the code
generator to do the conversions.

Fixes #50193

Change-Id: I24bdc09226c8e868f6282e0e4057ba6c3ad5c41a
Reviewed-on: https://go-review.googlesource.com/c/go/+/372514


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>