Ensure SROA gets the correct pointer type. (#2247)

We initially assumed that if the type manager returned the correct id
for the pointee type, that we would get the correct pointer type back,
but that is not true.  See the unit test added with this commit.  We
need to fall back to the linear search any time we are looking for a
pointer to a type that may not be unique.

At the same time, SROA considered an OpName on a variable to be a use of
the entire variable.  That has been fixed.

Fixes #2209.