[ARM][ConstantIslands] Correct MinNoSplitDisp calculation

MinNoSplitDisp was first introduced in D16890 to handle cases where the
ConstantIslands pass fails to converge in the presence of big basic
blocks. However, the computation of the variable seems to be wrong as it
currently computes the offset immediately following UserBB. In other words, it
represents the distance from the beginning of the function to the end of
UserBB. The distance from the beginning of the function does not seem to be a
good indicator of how big the basic block is unless the basic block is close to
the beginning of the function. I think MinNoSplitDisp should compute the
distance between UserOffset and the end of UserBB instead.

Author: pzhengqc

llvm/lib/Target/ARM/ARMConstantIslandPass.cpp

@@ -1324,7 +1324,8 @@ bool ARMConstantIslands::findAvailableWater(CPUser &U, unsigned UserOffset,
   MachineBasicBlock *UserBB = U.MI->getParent();
   BBInfoVector &BBInfo = BBUtils->getBBInfo();
   const Align CPEAlign = getCPEAlign(U.CPEMI);
-  unsigned MinNoSplitDisp = BBInfo[UserBB->getNumber()].postOffset(CPEAlign);
+  unsigned MinNoSplitDisp =
+      BBInfo[UserBB->getNumber()].postOffset(CPEAlign) - UserOffset;
   if (CloserWater && MinNoSplitDisp > U.getMaxDisp() / 2)
     return false;
   for (water_iterator IP = std::prev(WaterList.end()), B = WaterList.begin();;