FastISel needs to remove dead code when it bails out.

When FastISel fails to translate an instruction it hands off code
generation to SelectionDAG. Before it does so, it may have generated
local value instructions to feed phi nodes in successor blocks. These
instructions will then be generated again by SelectionDAG, causing
duplication and less efficient code, including extra spill
instructions.

Patch by Wolfgang Pieb!

Differential Revision: http://reviews.llvm.org/D11768

llvm-svn: 255520