[LSV] Fix the ContextInst for computeKnownBits.

Previously we used the later of GEPA or GEPB.  This is hacky because
really we should be using the later of the two load/store instructions
being considered.  But also it's flat-out incorrect, because GEPA and
GEPB might be in different BBs, in which case we cannot ask which one
comes last (assertion failure,
https://reviews.llvm.org/D149893#4378332).

Fixed, now we use the correct context instruction.

Differential Revision: https://reviews.llvm.org/D151630