Introduce PackedVector, useful for storing a vector of values using a specific...

Introduce PackedVector, useful for storing a vector of values using a specific number of bits for each
value. Both signed and unsigned types can be used, e.g

   PackedVector<signed, 2> vec;

will create a vector accepting values -2, -1, 0, 1. Any other value will hit an assertion.

llvm-svn: 132325