Swift Calling Convention: swifterror target-independent change.

At IR level, the swifterror argument is an input argument with type
ErrorObject**. For targets that support swifterror, we want to optimize it
to behave as an inout value with type ErrorObject*; it will be passed in a
fixed physical register.

The main idea is to track the virtual registers for each swifterror value. We
define swifterror values as AllocaInsts with swifterror attribute or a function
argument with swifterror attribute.

In SelectionDAGISel.cpp, we set up swifterror values (SwiftErrorVals) before
handling the basic blocks.

When iterating over all basic blocks in RPO, before actually visiting the basic
block, we call mergeIncomingSwiftErrors to merge incoming swifterror values when
there are multiple predecessors or to simply propagate them. There, we create a
virtual register for each swifterror value in the entry block. For predecessors
that are not yet visited, we create virtual registers to hold the swifterror
values at the end of the predecessor. The assignments are saved in
SwiftErrorWorklist and will be materialized at the end of visiting the basic
block.

When visiting a load from a swifterror value, we copy from the current virtual
register assignment. When visiting a store to a swifterror value, we create a
virtual register to hold the swifterror value and update SwiftErrorMap to
track the current virtual register assignment.

Differential Revision: http://reviews.llvm.org/D18108

llvm-svn: 265433