Reapply r113875 with additional cleanups.

Reapply r113875 with additional cleanups.

"The register specified for a dregpair is the corresponding Q register, so to
get the pair, we need to look up the sub-regs based on the qreg. Create a
lookup function since we don't have access to TargetRegisterInfo here to
be able to use getSubReg(ARM::dsub_[01])."

Additionaly, fix the NEON VLD1* and VST1* instruction patterns not to use
the dregpair modifier for the 2xdreg versions. Explicitly specifying the two
registers as operands is more correct and more consistent with the other
instruction patterns. This enables further cleanup of special case code in the
disassembler as a nice side-effect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@113903 91177308-0d34-0410-b5e6-96231b3b80d8