[RTG][Elaboration] Each virtual register should only be materialized once and every user should use it

[maerhart]

rtg.sequence @virtRegSeq(%reg: !rtgtest.ireg) {
  %imm = rtgtest.immediate #rtgtest.imm12<0>
  rtgtest.rv32i.jalr %reg, %reg, %imm
}

rtg.test @virtualRegisters() {
  %r0 = rtg.virtual_reg [#rtgtest.a0, #rtgtest.a1]
  %imm = rtgtest.immediate #rtgtest.imm12<0>
  rtgtest.rv32i.jalr %r0, %r0, %imm
  %s0 = rtg.get_sequence @virtRegSeq : !rtg.sequence<!rtgtest.ireg>
  %s1 = rtg.substitute_sequence %s0(%r0) : !rtg.sequence<!rtgtest.ireg>
  %s2 = rtg.randomize_sequence %s1
  rtg.embed_sequence %s2

Elaborates to

  rtg.test @virtualRegisters() {
    %0 = rtg.virtual_reg [#rtgtest.a0 : !rtgtest.ireg, #rtgtest.a1 : !rtgtest.ireg]
    %2 = rtgtest.immediate #rtgtest.imm12<0> : !rtgtest.imm12
    rtgtest.rv32i.jalr %0, %0, %2
    rtgtest.rv32i.jalr %0, %0, %2
    %3 = rtg.virtual_reg [#rtgtest.a0 : !rtgtest.ireg, #rtgtest.a1 : !rtgtest.ireg]
    %4 = rtgtest.immediate #rtgtest.imm12<0> : !rtgtest.imm12
    rtgtest.rv32i.jalr %3, %3, %4
}

The second register shouldn't be materialized, but instead the first one should be reused.