Fix SCEV forgetMemoizedResults should search and destroy backedge exprs.

Fixes PR15570: SEGV: SCEV back-edge info invalid after dead code removal.

Indvars creates a SCEV expression for the loop's back edge taken
count, then determines that the comparison is always true and
removes it.

When loop-unroll asks for the expression, it contains a NULL
SCEVUnknkown (as a CallbackVH).

forgetMemoizedResults should invalidate the loop back edges expression.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177986 91177308-0d34-0410-b5e6-96231b3b80d8
Manually update the dependencies in the Makefiles. It turns out that all
that work on the LLVMBuild based dependency specification didn't
actually work, we just now maintain dependencies in *3* places instead
of 2. Yay.

There may still be some missing dependencies, I'm still sifting through
the bots and my builds, but this is a step in the right direction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177988 91177308-0d34-0410-b5e6-96231b3b80d8