target/arm: Convert Neon 2-reg-scalar integer multiplies to decodetree

target/arm: Convert Neon 2-reg-scalar integer multiplies to decodetree

Convert the VMLA, VMLS and VMUL insns in the Neon "2 registers and a
scalar" group to decodetree.  These are 32x32->32 operations where
one of the inputs is the scalar, followed by a possible accumulate
operation of the 32-bit result.

The refactoring removes some of the oddities of the old decoder:
 * operands to the operation and accumulation were often
   reversed (taking advantage of the fact that most of these ops
   are commutative); the new code follows the pseudocode order
 * the Q bit in the insn was in a local variable 'u'; in the
   new code it is decoded into a->q

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>