[LSV] Refactoring + supporting bitcasts to a type of different size

This is mostly a preparation work for adding a limited support for
select instructions. It proved to be difficult to do due to size and
irregularity of Vectorizer::isConsecutiveAccess, this is fixed here I
believe.

It also turned out that these changes make it simpler to finish one of
the TODOs and fix a number of other small issues, namely:

1. Looking through bitcasts to a type of a different size (requires
careful tracking of the original load/store size and some math
converting sizes in bytes to expected differences in indices of GEPs).

2. Reusing partial analysis of pointers done by first attempt in proving
them consecutive instead of starting from scratch. This added limited
support for nested GEPs co-existing with difficult sext/zext
instructions. This also required a careful handling of negative
differences between constant parts of offsets.

3. Handing a case where the first pointer index is not an add, but
something else (a function parameter for instance).

I observe an increased number of successful vectorizations on a large
set of shader programs. Only few shaders are affected, but those that
are affected sport >5% less loads and stores than before the patch.

Reviewed By: rampitec

Differential-Revision: https://reviews.llvm.org/D49342
llvm-svn: 337489