[RISCV] Add shrinkwrap test cases showing gaps in current impl

This covers multiple interactions reduced from larger workloads:
1) Rematerializing addi s0, x0, <imm> to avoid the need to spill a CSR, with
   the common user being a branch.  (i.e. branch on immediate idioms)
2) Rematerializing addi s0, a0, <imm> to avoid the need to spill a CSR, with
   the common user being a vector load or store.  (i.e. because we don't
   have (reg+imm) addressing on vector.)
3) Independent of the previous, we could still shrink wrap these by locally
   using a non-CSR, and deferring the move into csr into the non-shrink
   wrapped path.
4) Weirdly, MachineCSE is producing a different result when an edge is
   manually split.  This edge split should be irrelevant to the CSE?

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