LoopUnroll: Clean up the maze of initialization for unroll parameters. NFC

LoopUnroll: Clean up the maze of initialization for unroll parameters. NFC

The layering of where the various loop unroll parameters are
initialized and overridden here was very confusing, making it pretty
difficult to tell just how the various sources interacted. Instead, we
put all of the initialization logic together in a single function so
that it's obvious what overrides what.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257426 91177308-0d34-0410-b5e6-96231b3b80d8