[Loads/SROA] Remove blatantly incorrect code and fix a bug revealed in the process

The code we had isSafeToLoadUnconditionally was blatantly wrong. This function takes a "Size" argument which is supposed to describe the span loaded from. Instead, the code use the size of the pointer passed (which may be unrelated!) and only checks that span. For any Size > LoadSize, this can and does lead to miscompiles.

Worse, the generic code just a few lines above correctly handles the cases which *are* valid. So, let's delete said code.

Removing this code revealed two issues:
1) As noted by jdoerfert the removed code incorrectly handled external globals.  The test update in SROA is to stop testing incorrect behavior.
2) SROA was confusing bytes and bits, but this wasn't obvious as the Size parameter was being essentially ignored anyway.  Fixed.

Differential Revision: https://reviews.llvm.org/D66778

llvm-svn: 370102