[ARM] Make PerformSHLSimplify add nodes to the DAG worklist correctly.

Intentionally excluding nodes from the DAGCombine worklist is likely
to lead to weird optimizations and infinite loops, so it's generally
a bad idea.

To avoid the infinite loops, fix DAGCombine to use the
isDesirableToCommuteWithShift target hook before performing the
transforms in question, and implement the target hook in the ARM
backend disable the transforms in question.

Fixes https://bugs.llvm.org/show_bug.cgi?id=38530 . (I don't have a
reduced testcase for that bug. But we should have sufficient test
coverage for PerformSHLSimplify given that we're not playing weird
tricks with the worklist. I can try to bugpoint it if necessary,
though.)

Differential Revision: https://reviews.llvm.org/D50667