Cleanup collectChangingRegs

The handling of the epilogue is best expressed as an early exit and
there is no reason to look for register defs in DbgValue MIs.

Patch by Frederic Riss!
Improve performance of calculateDbgValueHistory.

In r210492 the logic of calculateDbgValueHistory was changed to end
register variable live ranges at the end of MBB conditionally on
the fact that the register was or not clobbered by the function body.

This requires an initial scan of all the operands of the function
to collect all clobbered registers. In a second pass over all
instructions, we compare this set with the set of clobbered
registers for the current MachineInstruction. This modification
incurred a compilation time regression on some benchmarks: the
debug info emission phase takes ~10% more time.

While a small performance hit is unavoidable due to the initial
scan requirement, we can improve the situation by avoiding to
create too many temporary sets and just use lambdas to work directly
on the result of the initial scan.

Fixes <rdar://problem/17884104>

Patch by Frederic Riss!