[FuncSpec] Compute specialisation gain even when forcing specialisation

When rewriting the call sites to call the new specialised functions, a
single call site can be matched by two different specialisations - a
"less specialised" version of the function and a "more specialised"
version of the function, e.g.  for a function

    void f(int x, int y)

the call like `f(1, 2)` could be matched by either

    void f.1(int x /* int y == 2 */);

or

    void f.2(/* int x == 1, int y == 2 */);

The `FunctionSpecialisation` pass tries to match specialisation in the
order of decreasing gain, so "more specialised" functions are
preferred to "less specialised" functions. This breaks, however, when
using the flag `-force-function-specialization`, in which case the
cost/benefit analysis is not performed and all the specialisations are
equally preferable.

This patch makes the pass calculate specialisation gain and order the
specialisations accordingly even when `-force-function-specialization`
is used, under the assumption that this flag has purely debugging
purpose and it is reasonable to ignore the extra computing effort it
incurs.

Reviewed By: ChuanqiXu, labrinea

Differential Revision: https://reviews.llvm.org/D136180