[CodeGenPrep]Restructure promoting Ext to form ExtLoad

Summary:
Instead of just looking for a load which is mergable with Ext to form ExtLoad, trying to promote Exts as long as the cost is acceptable. This change is not a NFC as it continue promoting Exts even after finding a load during promotions; the change in arm64-codegen-prepare-extload.ll described in 2.b might show the case.
This change was motivated from D26524.  Based on this change, I will move the transformation performed in aarch64-type-promotion into CGP.

Reviewers: jmolloy, qcolombet, mcrosier, javed.absar

Reviewed By: qcolombet

Subscribers: rengolin, llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D27853

llvm-svn: 298114