Extend TableGen instruction selection matcher to improve handling

of complex instruction operands (e.g. address modes).

Currently, if a Pat pattern creates an instruction that has a complex
operand (i.e. one that consists of multiple sub-operands at the MI
level), this operand must match a ComplexPattern DAG pattern with the
correct number of output operands.

This commit extends TableGen to alternatively allow match a complex
operands against multiple separate operands at the DAG level.

This allows using Pat patterns to match pre-increment nodes like
pre_store (which must have separate operands at the DAG level) onto
an instruction pattern that uses a multi-operand memory operand,
like the following example on PowerPC (will be committed as a
follow-on patch):

  def STWU  : DForm_1<37, (outs ptr_rc:$ea_res), (ins GPRC:$rS, memri:$dst),
                    "stwu $rS, $dst", LdStStoreUpd, []>,
                    RegConstraint<"$dst.reg = $ea_res">, NoEncode<"$ea_res">;

  def : Pat<(pre_store GPRC:$rS, ptr_rc:$ptrreg, iaddroff:$ptroff),
            (STWU GPRC:$rS, iaddroff:$ptroff, ptr_rc:$ptrreg)>;

Here, the pair of "ptroff" and "ptrreg" operands is matched onto the
complex operand "dst" of class "memri" in the "STWU" instruction.

Approved by Jakob Stoklund Olesen.

llvm-svn: 177428