[ARM] Use searchable-table for banked registers

This is a continuation of https://reviews.llvm.org/D36219

This patch uses reverse mapping (encoding->name) in
ARMInstPrinter::printBankedRegOperand to get rid of
hard-coded values (as pointed out by @olista01).

Reviewed by: @fhahn, @rovka, @olista01
Differential Revision: https://reviews.llvm.org/D36260

llvm-svn: 310072