Rewrite pre-increment store patterns to use standard memory operands.

Currently, pre-increment store patterns are written to use two separate
operands to represent address base and displacement:

  stwu $rS, $ptroff($ptrreg)

This causes problems when implementing the assembler parser, so this
commit changes the patterns to use standard (complex) memory operands
like in all other memory access instruction patterns:

  stwu $rS, $dst

To still match those instructions against the appropriate pre_store
SelectionDAG nodes, the patch uses the new feature that allows a Pat
to match multiple DAG operands against a single (complex) instruction
operand.

Approved by Hal Finkel.