[GlobalISel][IRTranslator] Support PHI instructions in landingpad blocks

The check for the landingpad instructions was overly restrictive. In optimimized builds PHI nodes can appear
before the landingpad instructions, resulting in a fallback to SelectionDAG.

This change relaxes the check to allow PHI nodes.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D86141