[X86] Reduce complexity of the LEA optimization pass, by Andrey Turetsky.

In the OptimizeLEA pass keep instructions' positions in the basic block saved and use them for calculation of the distance between two instructions instead of std::distance. This reduces complexity of the pass from O(n^3) to O(n^2) and thus the compile time.
Differential Revision: http://reviews.llvm.org/D15692

llvm-svn: 257328