[mlir][tosa] Update the limit of 62 on shift.

[psunn]

Summary

Aligns the shift requirement with the TOSA specification.

Change-Id: Ia8881db0cac6632f5314c27c5036fd78c230ee1d

[eric-k256]

Looks good, matches what is in the TOSA specification.

mlir/lib/Dialect/Tosa/Utils/QuantUtils.cpp

static void computeMultiplierAndShiftTosaScale16(double scale,
   // shift bits.
   shift = (-shift) + 15;
 
   assert(shiftedM <= std::numeric_limits<int32_t>::max() &&
          "Shifted mantissa exceeds 32-bit signed output type");
 
   multiplier = static_cast<int32_t>(shiftedM);
 
-  // Shifting tops out at 63 bits. Right shift to make 63 bits the max.
-  if (shift > 63) {
+  // Shifting tops out at 62 bits. Right shift to make 62 bits the max.
+  // The limit of 62 on shift allows the shift to be decomposed as
+  // two right shifts of 31.
+  if (shift > 62) {
     // Shifting the multiplier by more than 31-bits is unnecessary.
-    multiplier = multiplier >> std::min<int32_t>(31, shift - 63);
-    shift = 63;
+    multiplier = multiplier >> std::min<int32_t>(31, shift - 62);
+    shift = 62;
   }
 }
 
 /// From a scale value, generates multiplier and shift values where
 /// mantissa is in [-1.0,-0.5] or [0.5, 1.0] such that
 /// multiplier = mantissa*2^shift for 32-bit scaling.
 static void computeMultiplierAndShiftTosaScale32(double scale,
                                                  int32_t &multiplier,
static void computeMultiplierAndShiftTosaScale32(double scale,
   // shift bits.
   shift = (-shift) + 31;
 
   assert(shiftedM <= std::numeric_limits<int32_t>::max() &&
          "Shifted mantissa exceeds 32-bit signed output type");
 
   multiplier = static_cast<int32_t>(shiftedM);
 
-  // Shifting tops out at 63 bits. Right shift to make 63 bits the max.
-  if (shift > 63) {
+  // Shifting tops out at 62 bits. Right shift to make 62 bits the max.
+  // The limit of 62 on shift allows the shift to be decomposed as
+  // two right shifts of 31.
+  if (shift > 62) {
     // Shifting the multiplier by more than 32-bits is unnecessary.
-    multiplier = multiplier >> std::min<int32_t>(31, shift - 63);
-    shift = 63;
+    multiplier = multiplier >> std::min<int32_t>(31, shift - 62);
+    shift = 62;
   }
 }
 
 /// Generates a quantized multiplier/shift from double.
 void mlir::tosa::computeMultiplierAndShift(double scale, int32_t &multiplier,
                                            int32_t &shift, int32_t scaleWidth) {
 
   switch (scaleWidth) {