[TableGen] Support multi-alternative pattern fragments

[TableGen] Support multi-alternative pattern fragments

A TableGen instruction record usually contains a DAG pattern that will
describe the SelectionDAG operation that can be implemented by this
instruction. However, there will be cases where several different DAG
patterns can all be implemented by the same instruction. The way to
represent this today is to write additional patterns in the Pattern
(or usually Pat) class that map those extra DAG patterns to the
instruction. This usually also works fine.

However, I've noticed cases where the current setup seems to require
quite a bit of extra (and duplicated) text in the target .td files.
For example, in the SystemZ back-end, there are quite a number of
instructions that can implement an "add-with-overflow" operation.
The same instructions also need to be used to implement just plain
addition (simply ignoring the extra overflow output). The current
solution requires creating extra Pat pattern for every instruction,
duplicating the information about which particular add operands
map best to which particular instruction.

This patch enhances TableGen to support a new PatFrags class, which
can be used to encapsulate multiple alternative patterns that may
all match to the same instruction.  It operates the same way as the
existing PatFrag class, except that it accepts a list of DAG patterns
to match instead of just a single one.  As an example, we can now define
a PatFrags to match either an "add-with-overflow" or a regular add
operation:

  def z_sadd : PatFrags<(ops node:$src1, node:$src2),
                        [(z_saddo node:$src1, node:$src2),
                         (add node:$src1, node:$src2)]>;

and then use this in the add instruction pattern:

  defm AR : BinaryRRAndK<"ar", 0x1A, 0xB9F8, z_sadd, GR32, GR32>;

These SystemZ target changes are implemented here as well.

Note that PatFrag is now defined as a subclass of PatFrags, which
means that some users of internals of PatFrag need to be updated.
(E.g. instead of using PatFrag.Fragment you now need to use
!head(PatFrag.Fragments).)

The implementation is based on the following main ideas:
- InlinePatternFragments may now replace each original pattern
  with several result patterns, not just one.
- parseInstructionPattern delays calling InlinePatternFragments
  and InferAllTypes.  Instead, it extracts a single DAG match
  pattern from the main instruction pattern.
- Processing of the DAG match pattern part of the main instruction
  pattern now shares most code with processing match patterns from
  the Pattern class.
- Direct use of main instruction patterns in InferFromPattern and
  EmitResultInstructionAsOperand is removed; everything now operates
  solely on DAG match patterns.

Reviewed by: hfinkel

Differential Revision: https://reviews.llvm.org/D48545

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336999 91177308-0d34-0410-b5e6-96231b3b80d8