[FuncSpec] Fix inconsistent treatment of global variables

There are a few inaccuracies with how FuncSpec handles global
variables.

When specialisation on non-const global variables is disabled (the
default) the pass could nevertheless perform some specializations,
e.g. on a constant GEP expression, or on a SSA variable, for which the
Solver has determined it has the value of a global variable.

When specialisation on non-const global variables is enabled, the pass
would skip non-scalars, e.g. a global array, but this should be
completely inconsequential, a pointer is a pointer.

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D149476

Change-Id: Ic73051b2f8602587306760bf2ec552e5860f8d39