Extend vector member references to include {.hi, .lo, .e, .o} which return a

vector of the same element type and half the width, with the high, low, even,
and odd elements respectively.

Allow member references to member references, so that .hi.hi gives you the high
quarter of a vector.  This is fairly convenient syntax for some insert/extract
operations.

Remove some unnecessary methods/types in the ExtVectorElementExpr class.

llvm-svn: 50892