[ARM] Add a register class for GPR pairs without SP and use it. NFCI

[ARM] Add a register class for GPR pairs without SP and use it. NFCI

Summary:
Currently Thumb2InstrInfo.cpp uses a register class which is
auto-generated by tablegen. Such approach is fragile because
auto-generated classes might change when other register classes are
added. For example, before https://reviews.llvm.org/D62667
we were using GPRPair_with_gsub_1_in_rGPRRegClass, but had to
change it to GPRPair_with_gsub_1_in_GPRwithAPSRnospRegClass
because the former class stopped being generated (this did not change
the functionality though).

This patch adds a register class consisting of even-odd GPR register
pairs from (R0, R1) to (R10, R11), which excludes (R12, SP) and uses
it in Thumb2InstrInfo.cpp instead of
GPRPair_with_gsub_1_in_GPRwithAPSRnospRegClass.

Reviewers: ostannard, simon_tatham, dmgreen, efriedma

Reviewed By: simon_tatham

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69026

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374990 91177308-0d34-0410-b5e6-96231b3b80d8