[Hexagon] Fix debug information for local objects

[Hexagon] Fix debug information for local objects

- Isolate the check for the existence of a stack frame into hasFP.
- Implement getFrameIndexReference for DWARF address computation.
- Use getFrameIndexReference for offset computation in eliminateFrameIndex.
- Preserve debug information for dynamically allocated stack objects.
- Prefer FP to access local objects at -O0.
- Add experimental code to skip allocframe when not strictly necessary
  (disabled by default).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250718 91177308-0d34-0410-b5e6-96231b3b80d8