[DebugInfo] Debug locations for constant SD nodes

[DebugInfo] Debug locations for constant SD nodes

Several updates for [DebugInfo] Add debug locations to constant SD nodes (r235989).
Includes:

 *  re-enabling the change (disabled recently);
 *  missing change for FP constants;
 *  resetting debug location of constant node if it's used more than at one place
    to prevent emission of wrong locations in case of coalesced constants;
 *  a couple of additional tests.

Now all look ups in CSEMap are wrapped by additional method.

Comment in D9084 suggests that debug locations aren't useful for "target constants",
so there might be one more change related to this API (namely, dropping debug
locations for getTarget*Constant methods).

Differential Revision: http://reviews.llvm.org/D9604

llvm-svn: 237237