Move vector bitcast handling in constant expressions from the expression

Move vector bitcast handling in constant expressions from the expression
evaluator into constant initializer handling / IRGen. The practical consequence
of this is that the bitcast now lives in the constant's definition, rather than
in its uses.

The code in the constant expression evaluator was producing vectors of the wrong
type and size (and possibly of the wrong value for a big-endian int-to-vector
bitcast). We were getting away with this only because we don't yet support
constant-folding of any expressions which inspect vector values.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@145981 91177308-0d34-0410-b5e6-96231b3b80d8