[VectorComine] Restrict single-element-store index to inbounds constant

Vector single element update optimization is landed in 2db4979c. But the
scope needs restriction. This patch restricts the index to inbounds and
vector must be fixed sized. In future, we may use value tracking to
relax constant restrictions.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D102146