[CleanUp] Remove NumInstructions field from LoopVectorizer's RegisterUsage struct.

[CleanUp] Remove NumInstructions field from LoopVectorizer's RegisterUsage struct.

Summary:
This variable is largely going unused; aside from reporting number of instructions for in DEBUG builds.

The only use of NumInstructions is in debug output to represent the LoopSize.  That value can be can be misleading as it also includes metadata instructions (e.g., DBG_VALUE) which have no real impact.  If we do choose to keep this around, we probably should guard it by a DEBUG macro, as it's not used in production builds.

Reviewers: majnemer, congh, rengolin

Reviewed By: rengolin

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D44495

llvm-svn: 327589