The test suite of LoopVectorize suffers from a coverage hole when types
mismatch, and runtime checks are needed, with a conflict redux. Fix this
coverage hole by adding tests.

When expanding SCEV adds to geps, transfer the nuw flag to the resulting
gep. (Note that this doesn't apply to IV increment GEPs, which go
through a different code path.)

RuntimePointerChecking::reset() is used to reset its state between
subsequent analysis invocations. Also reset CanUseDiffCheck to its
default (true). Otherwise it might have been set to false during a
previous analysis invocation, which unnecessarily pessimizes the
subsequent analysis invocations with a pruned set of dependences.

This is in line with the other fields being reset.

With commit https://reviews.llvm.org/D152366 I introduced functionality
that permitted the hoisting of runtime memory checks from a vectorised
inner loop to the preheader of the next outer-most loop. This is useful
for benchmarks like SPEC2017's x264 where the inner loop is vectorised
and only has a small trip count. In such cases the runtime memory checks
become expensive and since the checks never fail in the case of x264 it
makes sense to do this. However, this behaviour was controlled by the
flag -hoist-runtime-checks which was off by default.

This patch enables this flag by default for all targets, since I believe
this is a generally beneficial thing to do. I have tested this with
SPEC2017 and I see 2.3% and 2.6% improvements with x264 on neoverse-v1
and neoverse-n1, respectively. Similarly, I saw slight improvements in
the overall geomean on both machines. The only other notable changes
were a 1% drop in the roms benchmark, which was compensated for by a 1%
improvement in fotonik3d.

Instead of expanding the src/sink SCEV expressions and emitting an IR
sub to compute the difference, the subtraction can be directly be
performed by ScalarEvolution. This allows the subtraction to be
simplified by SCEV, which in turn can reduced the number of redundant
runtime check instructions generated.

It also allows to generate checks that are invariant w.r.t. an outer
loop, if he inner loop AddRecs have the same outer loop AddRec as start.

Add a test case where the AddRec for the pointers in the inner loop
have the AddRec of the outer loop as start value.

It is sufficient to subtract the start values (%dst, %src) of the outer
AddRecs. This simplification will be done in a follow-up commit.