Tablegen tries to infer this from the selection DAG patterns defined for
the instructions, but it can't always.
An instructive example is CLZr64. CLZr32 is correctly inferred to have
no side-effects, but the selection DAG pattern for CLZr64 is slightly
more complicated, and in particular the ctlz DAG node is not at the root
of the pattern. Thus tablegen can't infer that CLZr64 has no