[CodeGenPrep] move aarch64-type-promotion to CGP

Summary:
Move the aarch64-type-promotion pass within the existing type promotion framework in CGP.
This change also support forking sexts when a new sext is required for promotion.
Note that change is based on D27853 and I am submitting this out early to provide a better idea on D27853.

Reviewers: jmolloy, mcrosier, javed.absar, qcolombet

Reviewed By: qcolombet

Subscribers: llvm-commits, aemerson, rengolin, mcrosier

Differential Revision: https://reviews.llvm.org/D28680

llvm-svn: 299379