Teach getCastOpcode about element-by-element vector casts. For example, "trunc"

can be used to turn a <4 x i64> into a <4 x i32> but getCastOpcode would assert
if you passed these types to it.  Note that this strictly extends the previous
functionality: if getCastOpcode previously accepted two vector types (i.e. didn't
assert) then it still will and returns the same opcode (BitCast).  That's because
before it would only accept vectors with the same bitwidth, and the new code only
touches vectors with the same length.  However if two vectors have both the same
bitwidth and the same length then their element types have the same bitwidth, so
the new logic will return BitCast as before.

llvm-svn: 131530