[PPC] Fix assertion failure during binary encoding with -mcpu=pwr9

Summary
clang -c -mcpu=pwr9 test/CodeGen/PowerPC/build-vector-tests.ll causes an assertion failure during the binary encoding.
The failure occurs when a D-form load instruction takes two register operands instead of a register + an immediate.

This patch fixes the problem and also adds an assertion to catch this failure earlier before the binary encoding (i.e. during lit test).
The fix is from Nemanja Ivanovic @nemanjai.

Differential Revision: https://reviews.llvm.org/D33482

llvm-svn: 304133