[SV] Canonicalizer ASAN failure

[dtzSiFive]

Input I had locally presently causes ASAN failure w/canonicalizer.

Input:

hw.module @Test(in %x : i8, in %clock: i1) {
  // All of these are equivalent to `var logic x` .

  // logic-type variable, assign to x.
  %logicvar = sv.logic : !hw.inout<i8>
  sv.assign %logicvar, %x : i8

  // Same but use keyword "reg" (reg x; = var logic x;)
  %regvar = sv.reg : !hw.inout<i8>
  sv.assign %regvar, %x : i8

  // sv.reg supports initialization operand, demonstrate.
  %regwithinit = sv.reg init %x : !hw.inout<i8>
}

ASAN report snippet:

=================================================================
==4169766==ERROR: AddressSanitizer: heap-use-after-free on address 0x50d000000640 at pc 0x564cb22e62ba bp 0x7ffd4dcf1670 sp 0x7ffd4dcf1668
READ of size 8 at 0x50d000000640 thread T0
    #0 0x564cb22e62b9 in mlir::detail::IROperandBase::getNextOperandUsingThisValue() /home/will/src/sifive/circt/llvm/llvm/../mlir/include/mlir/IR/UseDefLists.h:43:58
    #1 0x564cb22e62b9 in mlir::ValueUseIterator<mlir::OpOperand>::operator++() /home/will/src/sifive/circt/llvm/llvm/../mlir/include/mlir/IR/UseDefLists.h:322:39
    #2 0x564cb22e62b9 in circt::sv::RegOp::canonicalize(circt::sv::RegOp, mlir::PatternRewriter&) /home/will/src/sifive/circt/lib/Dialect/SV/SVOps.cpp:311:18

At a glance, we're deleting users while iterating over them (and may delete operations using this more than once twice / invalidate the list).