TableGen: Add a defset statement

TableGen: Add a defset statement

Allows capturing a list of concrete instantiated defs.

This can be combined with foreach to create parallel sets of def
instantiations with less repetition in the source. This purpose is
largely also served by multiclasses, but in some cases multiclasses
can't be used.

The motivating example for this change is having a large set of
intrinsics, which are generated from the IntrinsicsBackend.td file
included by Intrinsics.td, and a corresponding set of instruction
selection patterns, which are generated via the backend's .td files.

Multiclasses cannot be used to eliminate the redundancy in this case,
because a multiclass cannot span both LLVM's common .td files and
the backend .td files at the same time.

Change-Id: I879e35042dceea542a5e6776fad23c5e0e69e76b

Differential revision: https://reviews.llvm.org/D44109

llvm-svn: 327121