[Comb] Concat/extract canonicalizer crashes on loop

Nobody has claimed this yet.

Consider the following input:
```mlir
hw.module @Foo(in %a : i1, in %b : i1, out z : i4) {
%0 = comb.extract %1 from 2 : (i4) -> i2
%1 = comb.concat %0, %b, %a : i2, i1, i1
hw.output %1 : i4
}
```
Running this through `circt-opt --canonicalize` crashes:
```
Assertion `op->use_empty() && "expected 'op' to have no uses"' failed.
[...]
#8 mlir::RewriterBase::eraseOp(mlir::Operation*)
#9 mlir::RewriterBase::replaceOp(mlir::Operation*, mlir::ValueRange)
#10 circt::replaceOpAndCopyNamehint(mlir::PatternRewriter&, mlir::Operation*, mlir::Value)
#11 extractConcatToConcatExtract(circt::comb::ExtractOp, circt::comb::ConcatOp, mlir::PatternRewriter&)
/home/fabian/code/circt/lib/Dialect/Comb/CombFolds.cpp:513:3
#12 circt::comb::ExtractOp::canonicalize(circt::comb::ExtractOp, mlir::PatternRewriter&)
/home/fabian/code/circt/lib/Dialect/Comb/CombFolds.cpp:578:12
```
The input was generate from (loop is useless in practice but intentional):
```systemverilog
module Foo(input logic a, logic b, output logic [3:0] z);
logic [3:0] x;
always_comb begin
x[0] = a;
x[1] = b;
end
assign z = x;
endmodule
```

The crash is in lib/Dialect/Comb/CombFolds.cpp, in extractConcatToConcatExtract around line 513, reached from ExtractOp::canonicalize. Run the supplied MLIR through circt-opt --canonicalize and inspect how the concat/extract loop is rewritten and erased. Done means this reproducer no longer asserts, with regression coverage for the case.

Written by the indexing model from the issue text.