Don't remove ref temps for PRIM_SET_MEMBER

Ref temps for PRIM_SET_MEMBER are always needed.
Merge pull request #379 from Kyle-B/ref-temp-set-member-fix

Don't remove ref temps for PRIM_SET_MEMBER

A very uncommon case in the compiler causes a ref temp to be generated for PRIM_SET_MEMBER. Any time this occurs, removing the ref temp will cause incorrect behavior.

This fixes the regression for `test/functions/diten/refArgIter.chpl` which was only failing in baseline testing (since we run `--baseline --inline` ref temp removal is turned on).