Do not rely on the enum values of argument registers A0-A3 being consecutive.

Do not rely on the enum values of argument registers A0-A3 being consecutive.

Define function getNextIntArgReg, which takes a register as a parameter and
returns the next O32 argument integer register. Use this function when double
precision floating point arguments are passed in two integer registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@140363 91177308-0d34-0410-b5e6-96231b3b80d8