PseudoSourceValue: Introduce a 'PSVKind' enumerator.

This commit introduces a new enumerator named 'PSVKind' in the
'PseudoSourceValue' class. This enumerator is now used to distinguish between
the various kinds of pseudo source values.

This change is done in preparation for the changes to the pseudo source value
object management and to the PseudoSourceValue's class hierarchy - the next two
PseudoSourceValue commits will get rid of the global variable that manages the
pseudo source values and the mips specific MipsCallEntry subclass.

Reviewers: Akira Hatanaka
llvm-svn: 244687