Fast-ISel: Remove dead code after falling back from selecting call instructions (PR20863)

Previously, fast-isel would not clean up after failing to select a call
instruction, because it would have called flushLocalValueMap() which moves
the insertion point, making SavedInsertPt in selectInstruction() invalid.

Fixing this by making SavedInsertPt a member variable, and having
flushLocalValueMap() update it.

This removes some redundant code at -O0, and more importantly fixes PR20863.

Differential Revision: http://reviews.llvm.org/D5249