[JumpThreading][NFC] Rename LoadInst variables

Summary:
The JumpThreading pass has several locations where to the variable name LI
refers to a LoadInst type. This is confusing and inhibits the ability to use
LI for LoopInfo as a member of the JumpThreading class. Minor formatting
and comments were also altered to reflect this change.

Reviewers: dberlin, kuba, spop, sebpop

Reviewed by: sebpop

Subscribers: sebpop, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D42601

llvm-svn: 323695