[DOC] Fix `load` instructions' syntax, function definition.

Summary: In this patch, I updated `load` instruction syntax and fixed function definition. Besides, I re-named some variables to make them obey SSA rule.

Reviewers: MaskRay

Reviewed By: MaskRay

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63186

llvm-svn: 363142