Simplify SplitVecRes_UnaryOp by removing all the code that is

Simplify SplitVecRes_UnaryOp by removing all the code that is
trying to legalize the operand types when only the result type
is required to be legalized - the type legalization machinery
will get round to the operands later if they need legalizing.
There can be a point to legalizing operands in parallel with
the result: when this saves compile time or results in better
code.  There was only one case in which this was true: when
the operand is also split, so keep the logic for that bit.
As a result of this change, additional operand legalization
methods may need to be introduced to handle nodes where the
result and operand types can differ, like SIGN_EXTEND, but
the testsuite doesn't contain any tests where this is the case.
In any case, it seems better to require such methods (and die
with an assert if they doesn't exist) than to quietly produce
wrong code if we forgot to special case the node in
SplitVecRes_UnaryOp.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143026 91177308-0d34-0410-b5e6-96231b3b80d8