Mark values as trivially dead when their only use is a start or end lifetime intrinsic.

Summary:
If the only use of a value is a start or end lifetime intrinsic then mark the intrinsic as trivially dead. This should allow for that value to then be removed as well.

Currently, this only works for allocas, globals, and arguments.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79355