[dev.typeparams] cmd/compile: simplify autotmpname

Rather than manually formatting a byte-string and then using a map
lookup to convert it to string, we can just use a slice. This avoids
both the overhead of formatting the byte slice and the map lookup.

Change-Id: Ia7b883632ea990ce9ee848dd4b4e4cdfbd611212
Reviewed-on: https://go-review.googlesource.com/c/go/+/332191


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>