Fixed the implementations of these builtin functions for targets that don't have native masked load instructions so that they do no loads if the vector mask is all off, and only do an (unaligned) vector load if both the first and last element of the mask are on. Otherwise they serialize and do scalar loads for only the active lanes. This fixes a number of potential sources of crashes due to accessing invalid memory.
Add much more suppport for doubles and in64 types in the standard library, basically supporting everything for them that are supported for floats and int32s. (The notable exceptions being the approximate rcp() and rsqrt() functions, which don't really have sensible analogs for doubles (or at least not built-in instructions).)
