[Simplify] Actually remove unsed instruction from region header.

[Simplify] Actually remove unsed instruction from region header.

Since r312249 instructions of a entry block of region statements are
not marked as root anymore and hence can theoretically be removed
if unused. Theoretically, because the instruction list was not changed.

Still, MemoryAccesses for unused instructions were removed. This lead
to a failed assertion in the code generator  when the MemoryAccess for
the still listed instruction was not found.

This hould fix the
Assertion failed: ArrayAccess && "No array access found for instruction!",
file ScopInfo.h, line 1494
compiler crashes.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@312566 91177308-0d34-0410-b5e6-96231b3b80d8