Clamped vector load

@abadams @jrk

Code for clamped vector load is now in CodeGen_X86. The visitor for load ops is a duplicate of the one in CodeGen.cpp, with an additional parameter and helper function to do the initial call and prevent infinite recursion. There are also a few different helper functions that manipulate the IR; one to extract a clamped index as a ramp for the case where the limits of the ramp are within the bounds, and another to generate the bounds check that differentiates that case.