[tablegen][globalisel] Add support for nested instruction matching.

Summary:
Lift the restrictions that prevented the tree walking introduced in the
previous change and add support for patterns like:
  (G_ADD (G_MUL (G_SEXT $src1), (G_SEXT $src2)), $src3) -> SMADDWrrr $dst, $src1, $src2, $src3
Also adds support for G_SEXT and G_ZEXT to support these cases.

One particular aspect of this that I should draw attention to is that I've
tried to be overly conservative in determining the safety of matches that
involve non-adjacent instructions and multiple basic blocks. This is intended
to be used as a cheap initial check and we may add a more expensive check in
the future. The current rules are:
* Reject if any instruction may load/store (we'd need to check for intervening
  memory operations.
* Reject if any instruction has implicit operands.
* Reject if any instruction has unmodelled side-effects.
See isObviouslySafeToFold().

Reviewers: t.p.northover, javed.absar, qcolombet, aditya_nandakumar, ab, rovka

Reviewed By: ab

Subscribers: igorb, dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D30539

llvm-svn: 299430