[LoopReroll] Fix rerolling loop with use outside the loop

Fixes PR41696

The loop-reroll pass generates an invalid IR (or its assertion
fails in debug build) if values of the base instruction and
other root instructions (terms used in the loop-reroll pass)
are used outside the loop block. See IRs written in PR41696
as examples.

The current implementation of the loop-reroll pass can reroll
only loops that don't have values that are used outside the
loop, except reduced values (the last values of reduction chains).
This is described in the comment of the `LoopReroll::reroll`
function.
https://github.com/llvm/llvm-project/blob/llvmorg-10.0.0/llvm/lib/Transforms/Scalar/LoopRerollPass.cpp#L1600

This is checked in the `LoopReroll::DAGRootTracker::validate`
function.
https://github.com/llvm/llvm-project/blob/llvmorg-10.0.0/llvm/lib/Transforms/Scalar/LoopRerollPass.cpp#L1393

However, the base instruction and other root instructions skip
this check in the validation loop.
https://github.com/llvm/llvm-project/blob/llvmorg-10.0.0/llvm/lib/Transforms/Scalar/LoopRerollPass.cpp#L1229

Moving the check in front of the skip is the logically simplest
fix. However, inserting the check in an earlier stage is better
in terms of compilation time of unrerollable loops. This fix
inserts the check for the base instruction into the function
to validate possible base/root instructions. Check for other
root instructions is unnecessary because they don't match any
base instructions if they have uses outside the loop.

Differential Revision: https://reviews.llvm.org/D79549