cmd/compile: fix SSA type comparison

A typo in the conversion code caused comparisons of SSA types to
report CMPeq when they were not in fact equal.

Fixes #40837

Change-Id: I0627eee51d524a585908b34a4590bc533c8415fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/248781


Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>