Silence various build warnings from Hexagon backend that show up in release builds...

Silence various build warnings from Hexagon backend that show up in release builds. Mostly converting 'assert(0)' to 'llvm_unreachable' to silence warnings about missing returns. Also fold some variable declarations into asserts to prevent the variables from being unused in release builds.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154660 91177308-0d34-0410-b5e6-96231b3b80d8