cmd/compile: make a type-specialized copy of encoding/binary.ReadUvarint

cmd/compile: make a type-specialized copy of encoding/binary.ReadUvarint

This is to get better escape analysis for a frequently-allocated object.

Change-Id: I1942a4e1c3cd1f0fa870bc8433a0165c18ce9c75
Reviewed-on: https://go-review.googlesource.com/c/go/+/435336
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>