[dev.regabi] cmd/compile: stop reusing Ntype for OSLICELIT length

For OSLICELITs, we were reusing the Ntype field after type checking to
hold the length of the OSLICELIT's backing array. However, Ntype is
only meant for nodes that can represent types. Today, this works only
because we currently use Name for all OLITERAL constants (whether
declared or not), whereas we should be able to represent them more
compactly with a dedicated type that doesn't implement Ntype.

Passes buildall w/ toolstash -cmp.

Change-Id: I385f1d787c41b016f507a5bad9489d59ccfde7f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/279152


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>