Change the way the lint sanity checking pass detects misaligned memory accesses.

Previously it was only be able to detect problems if the pointer was a numerical
value (eg inttoptr i32 1 to i32*), but not if it was an alloca or globa.  The
reason was the use of ComputeMaskedBits: imagine you have "alloca i8, align 2",
and ask ComputeMaskedBits what it knows about the bits of the alloca pointer.
It can tell you that the bottom bit is known zero (due to align 2) but it can't
tell you that bit 1 is known one.  That's because the address could be an even
multiple of 2 rather than an odd multiple, eg it might be a multiple of 4.  Thus
trying to use KnownOne is ineffective in the case of an alloca as it will never
have any bits set.  Instead look explicitly for constant offsets from allocas
and globals.

llvm-svn: 164595