go/ssa: make Builtin capable of representing non-spec-defined intrinsics.

Also, define ssa:wrapnilchk intrinsic to check and gracefully
fail when a T method is dynamically invoked via a nil *T receiver.
+ Test.

A follow-up CL will add another intrinsic, ssa:memclr.

+ minor cleanups.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/101170044