[AMDGPU][SplitModule] Fix unintentional integer division

A static analysis tool warned that a division was always being performed
in integer division, so was either 0.0 or 1.0. This doesn't seem
intentional, so has been fixed to return a true ratio using
floating-point division. This has a knock-on effect on how a test was
splitting modules.

Author: frasercrmck

llvm/lib/Target/AMDGPU/AMDGPUSplitModule.cpp

@@ -1101,7 +1101,7 @@ void RecursiveSearchSplitting::pickPartition(unsigned Depth, unsigned Idx,
         // Check if the amount of code in common makes it worth it.
         assert(SimilarDepsCost && Entry.CostExcludingGraphEntryPoints);
         const double Ratio =
-            SimilarDepsCost / Entry.CostExcludingGraphEntryPoints;
+            (double)SimilarDepsCost / Entry.CostExcludingGraphEntryPoints;
         assert(Ratio >= 0.0 && Ratio <= 1.0);
         if (LargeFnOverlapForMerge > Ratio) {
           // For debug, just print "L", so we'll see "L3=P3" for instance, which

llvm/test/tools/llvm-split/AMDGPU/large-kernels-merging.ll

@@ -15,19 +15,20 @@
 ; Also check w/o large kernels processing to verify they are indeed handled
 ; differently.
 
-; P0 is empty
-; CHECK0: declare
+; CHECK0: define internal void @HelperC()
+; CHECK0: define amdgpu_kernel void @C
 
-; CHECK1: define internal void @HelperC()
-; CHECK1: define amdgpu_kernel void @C
+; CHECK1: define internal void @large2()
+; CHECK1: define internal void @large1()
+; CHECK1: define internal void @large0()
+; CHECK1: define internal void @HelperB()
+; CHECK1: define amdgpu_kernel void @B
 
 ; CHECK2: define internal void @large2()
 ; CHECK2: define internal void @large1()
 ; CHECK2: define internal void @large0()
 ; CHECK2: define internal void @HelperA()
-; CHECK2: define internal void @HelperB()
 ; CHECK2: define amdgpu_kernel void @A
-; CHECK2: define amdgpu_kernel void @B
 
 ; NOLARGEKERNELS-CHECK0: define internal void @HelperC()
 ; NOLARGEKERNELS-CHECK0: define amdgpu_kernel void @C