[AArch64] Multiply extended 32-bit ints with `[U|S]MADDL'

During instruction selection, the AArch64 backend can recognise the
following pattern and generate an [U|S]MADDL instruction, i.e. a
multiply of two 32-bit operands with a 64-bit result:

(mul (sext i32), (sext i32))
However, when one of the operands is constant, the sign extension
gets folded into the constant in SelectionDAG::getNode(). This means
that the instruction selection sees this:

(mul (sext i32), i64)
...which doesn't match the pattern. Sign-extension and 64-bit
multiply instructions are generated, which are slower than one 32-bit
multiply.

Add a pattern to match this and generate the correct instruction, for
both signed and unsigned multiplies.

Patch by Chris Diamand!

llvm-svn: 259800