Tighten up checking of the validity of casts. (1) The IR parser would

happily accept things like "sext <2 x i32> to <999 x i64>".  It would
also accept "sext <2 x i32> to i64", though the verifier would catch
that later.  Fixed by having castIsValid check that vector lengths match
except when doing a bitcast.  (2) When creating a cast instruction, check
that the cast is valid (this was already done when creating constexpr
casts).  While there, replace getScalarSizeInBits (used to allow more
vector casts) with getPrimitiveSizeInBits in getCastOpcode and isCastable
since vector to vector casts are now handled explicitly by passing to the
element types; i.e. this bit should result in no functional change.

llvm-svn: 131532