[GISel] Fix match tree emitter.

The following changes are necessasy to get the generated tree
matcher to compile:

- In CodeExpansions::declare(), the assert() prevents connecting
  two instructions. E.g. the match code
    (match (MUL $t, $s1, $s2),
           (SUB $d, $t, $s3)),
  results in two declarations of $t, one for the def and one for
  the use. Removing the assertion allows this construct.
  If $t is later used, it is one of the operands, which should be
  perfectly fine.
- The code emitted in GIMatchTreeVRegDefPartitioner::generatePartitionSelectorCode()
  is not compilable:
  - The value of NewInstrID should be emitted, not the name
  - Both calls involving getOperand() end with one parenthesis too many
- Swaps generated condition for the partition code in the latter function

It also changes the rules i2p_to_p2i, fabs_fabs_fold, and fneg_fneg_fold
to use the tree matcher for a linear match. These rules are tested by:

CodeGen/AArch64/GlobalISel/combine-fabs.mir
CodeGen/AArch64/GlobalISel/combine-fneg.mir
CodeGen/AArch64/GlobalISel/combine-ptrtoint.mir
CodeGen/AMDGPU/GlobalISel/combine-add-nullptr.mir

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D133257