This PR instruments the optimization passes in the SystemZ backend with
calls to `MachineFunction::substituteDebugValuesForInst` where
instruction substitutions are made to instructions that may compute
tracked values.

Tests are also added for each of the substitutions that were inserted.
Details on the individual passes follow.

### systemz-copy-physregs
When a copy targets an access register, we redirect the copy via an
auxiliary register. This leads to the final result being written by a
newly inserted SAR instruction, rather than the original MI, so we need
to update the debug value tracking to account for this.

### systemz-long-branch
This pass relaxes relative branch instructions based on the actual
locations of blocks. Only one of the branch instructions qualifies for
debug value tracking: BRCT, i.e. branch-relative-on-count, which
subtracts 1 from a register and branches if the result is not zero. This
is relaxed into an add-immediate and a conditional branch, so any
`debug-instr-number` present must move to the add-immediate instruction.

### systemz-post-rewrite
This pass replaces `LOCRMux` and `SELRMux` pseudoinstructions with
either the real versions of those instructions, or with branching
programs that implement the intent of the Pseudo. In all these cases,
any `debug-instr-number` attached to the pseudo needs to be reallocated
to the appropriate instruction in the result, either LOCR, SELR, or a
COPY.

### systemz-elim-compare
Similar to systemz-long-branch, for this pass, only few substitutions
are necessary, since it mainly deals with conditional branch
instructions. The only exceptiona are again branch-relative-on-count, as
it modifies a counter as part of the instruction, as well as any of the
load instructions that are affected.