[AVR] Use correct register class for mul instructions

A number of multiplication instructions (muls, mulsu, fmul, fmuls,
fmulsu) had the wrong register class for an operand. This resulted in
the wrong register being used for the instruction.

Example:

    target datalayout = "e-P1-p:16:8-i8:8-i16:8-i32:8-i64:8-f32:8-f64:8-n8-a:8"
    target triple = "avr-atmel-none"

    define i16 @sliceAppend(i16, i16, i16, i16, i16, i16) addrspace(1) {
      %d = mul i16 %0, %5
      ret i16 %d
    }

The first instruction would be muls r24, r31 before this patch. The r31
should have been r15 if you look at the intermediate forms during
instruction selection / register allocation, but the generated
instruction uses r31. After this patch, an extra movw is inserted to get
%5 in range for muls.

To make sure this bug is fixed everywhere, I checked all instructions
and found that most multiplication instructions suffered from this bug,
which I have fixed with this patch. No other instructions appear to be
affected.

Differential Revision: https://reviews.llvm.org/D74281