go/ssa: use TypeSetOf directly; fix indexMode.meet

This CL removes the typeset iterator and its wrappers underIs and
typeSetIsEmpty, replacing each use with a direct loop over the terms
of typeparams.TypeSetOf, simplified using knowledge of each caller:

- isBytestring, nillable, isBasicConvTypes, soleTypeKind: loop over
  TypeSetOf(t).UnderlyingTypes(), without the f(nil) convention
  for type sets with no specific terms.
- emitConv: in well-typed code neither type set is empty or
  unrestricted and classify never returns zero, so the nil checks,
  early exit, and final panic were dead. The common case of a
  conversion not involving type parameters (99.6% across the
  tests) now avoids TypeSetOf entirely.
- indexType: in well-typed code all terms are non-interface types
  with identical element types, and are all maps or all non-maps.

Also, fix a bug in indexMode.meet, which returned the max, not
min (most constrained) mode as documented.  So for a type
parameter whose type set mixed arrays or strings with slices, such as
~[2]byte|~[]byte or ~string|~[]byte, indexType reported ixVar, and
the builder emitted IndexAddr on a non-addressable value (even a
string). meet is replaced by min.

Change-Id: Ief87474a16361bfd834c7f570d7944a8c9cf7a66
Reviewed-on: https://go-review.googlesource.com/c/tools/+/850790
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>