[ARM] Attempt to make Tail predication / RDA more resilient to empty blocks

There are a number of places in RDA where we assume the block will not
be empty. This isn't necessarily true for tail predicated loops where we
have removed instructions. This attempt to make the pass more resilient
to empty blocks, not casting pointers to machine instructions where they
would be invalid.

The test contains a case that was previously failing, but recently been
hidden on trunk. It contains an empty block to begin with to show a
similar error.

Differential Revision: https://reviews.llvm.org/D88926