[DAGCombiner][X86] Fold sra (sub AddC, (shl X, N1C)), N1C --> sext (sub...

[DAGCombiner][X86] Fold sra (sub AddC, (shl X, N1C)), N1C --> sext (sub AddC1',(trunc X to (width - N1C)))

We already handled this case for add with a constant RHS. A
similar pattern can occur for sub with a constant left hand side.

Test cases use add and a mul representing (neg (shl X, C)) because
that's what I saw in the wild. The mul will be decomposed and then
the new transform can kick in.

Tests have not been committed, but this patch shows the changes.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D128769