[RISCV][NFC] Use TargetRegisterClass::hasSubClassEq in storeRegToStackSlot/loadReadFromStackSlot
[RISCV][NFC] Use TargetRegisterClass::hasSubClassEq in storeRegToStackSlot/loadReadFromStackSlot

Simply checking for register class equality will break once additional 
register classes are added (as is done for the RVC instruction set extension).


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@320036 91177308-0d34-0410-b5e6-96231b3b80d8

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