[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

Author: aykevl

llvm/lib/Target/AVR/AVRInstrInfo.td

@@ -555,36 +555,36 @@ Defs = [R1, R0, SREG] in
 
     def MULSRdRr : FMUL2RdRr<0,
                              (outs),
-                             (ins GPR8:$lhs, GPR8:$rhs),
+                             (ins LD8:$lhs, LD8:$rhs),
                              "muls\t$lhs, $rhs",
                              []>,
                    Requires<[SupportsMultiplication]>;
   }
 
   def MULSURdRr : FMUL2RdRr<1,
                             (outs),
-                            (ins GPR8:$lhs, GPR8:$rhs),
+                            (ins LD8lo:$lhs, LD8lo:$rhs),
                             "mulsu\t$lhs, $rhs",
                             []>,
                   Requires<[SupportsMultiplication]>;
 
   def FMUL : FFMULRdRr<0b01,
                        (outs),
-                       (ins GPR8:$lhs, GPR8:$rhs),
+                       (ins LD8lo:$lhs, LD8lo:$rhs),
                        "fmul\t$lhs, $rhs",
                        []>,
              Requires<[SupportsMultiplication]>;
 
   def FMULS : FFMULRdRr<0b10,
                         (outs),
-                        (ins GPR8:$lhs, GPR8:$rhs),
+                        (ins LD8lo:$lhs, LD8lo:$rhs),
                         "fmuls\t$lhs, $rhs",
                         []>,
               Requires<[SupportsMultiplication]>;
 
   def FMULSU : FFMULRdRr<0b11,
                          (outs),
-                         (ins GPR8:$lhs, GPR8:$rhs),
+                         (ins LD8lo:$lhs, LD8lo:$rhs),
                          "fmulsu\t$lhs, $rhs",
                          []>,
                Requires<[SupportsMultiplication]>;