[AArch64] Restore codegen for AArch64 Cortex-A72/A73 after NFCI

Summary:
Code generation for Cortex-A72/Cortex-A73 was accidentally changed
by r271555, which was a NFCI. The isCortexA57() predicate was not true
for Cortex-A72/Cortex-A73 before r271555 (since it was checking the CPU
string). Because Cortex-A72/Cortex-A73 inherit all features from Cortex-A57,
all decisions previously guarded by isCortexA57() are now taken.

This change restores the behaviour before r271555 by adding separate
ProcA72/ProcA73, which have the required features to preserve code
generation.

Reviewers: kristof.beyls, aadg, mcrosier, rengolin

Subscribers: mcrosier, llvm-commits, aemerson, t.p.northover, MatzeB, rengolin

Differential Revision: http://reviews.llvm.org/D21182

llvm-svn: 273277