[mlir][vector][nfc] Rename index optimizations option

We are using "enable-index-optimizations" and "indexOptimizations" as
names for an optimization that consists of using i32 for indices within
a vector. For instance, when building a vector comparison for mask
generation. The name is confusing and suggests a scope beyond these
vector indices.  This change makes the function of the option explicit
in its name.

Differential Revision: https://reviews.llvm.org/D122415