Revert "[ARM] Improve codegen of volatile load/store of i64"

This reverts commit 8a125532.

A bug has been found when generating code for Thumb2. In some very
specific cases, the prologue/epilogue emitter generates erroneous stack
offsets for the new LDRD instructions that access the stack.

This bug does not seem to be caused by the reverted patch though. Likely
the latter has made an undiscovered issue emerge in the
prologue/epilogue emission pass. Nevertheless, this reversion is
necessary since it is blocking users of the ARM backend.