[RISCV] Fix SiFive7 formula for Reductions and ordered Reductions

Reductions specified the correct formula in the comments but was
implemented incorrectly.

Ordered reductions formula was refined to be more accurate to actual
hardware.

A llvm-mca test case for reductions is added so that we have a better
idea of how the model is performing.

Author: michaelmaitland

llvm/lib/Target/RISCV/RISCVSchedSiFive7.td

@@ -154,12 +154,12 @@ class SiFive7GetReductionCycles<string mx, int sew> {
     !eq(mx, "MF8") : 1
   );
   int c = !add(
-    !div(TwoTimesLMUL, DLEN),
+    TwoTimesLMUL,
     !mul(5, !add(4, !logtwo(!div(DLEN, sew))))
   );
 }
 
-/// Cycles for ordered reductions take approximatley 5*VL cycles
+/// Cycles for ordered reductions take approximatley 6*VL cycles
 class SiFive7GetOrderedReductionCycles<string mx, int sew> {
   defvar VLEN = 512;
   // (VLEN * LMUL) / SEW
@@ -172,7 +172,7 @@ class SiFive7GetOrderedReductionCycles<string mx, int sew> {
     !eq(mx, "MF4") : !div(!div(VLEN, 4), sew),
     !eq(mx, "MF8") : !div(!div(VLEN, 8), sew),
   );
-  int c = !mul(5, VLUpperBound);
+  int c = !mul(6, VLUpperBound);
 }
 
 class SiFive7AnyToGPRBypass<SchedRead read, int cycles = 2>