InstSimplify: shl nsw/nuw undef, %V -> undef

We can always choose an value for undef which might cause %V to shift
out an important bit except for one case, when %V is zero.

However, shl behaves like an identity function when the right hand side
is zero.
[ASan] Re-structure the allocator code. NFC.

Introduce "Allocator" object, which contains all the bits and pieces
ASan allocation machinery actually use: allocator from sanitizer_common,
quarantine, fallback allocator and quarantine caches, fallback mutex.

This step is a preparation to adding more state to this object. We want
to reduce dependency of Allocator on commandline flags and be able to
"safely" modify its behavior (such as the size of the redzone) at
runtime.