[ARM] Fix non-determenistic behaviour

Summary:
ARM Type Promotion pass does not clear
the container that defines if one variable
was visited or not, missing optimization
opportunities by luck when two llvm:Values
from different functions  are allocated at
the same memory address.

Also fixes a comment and uses existing
method to pop and obtain last element
of the worklist.

Reviewers: samparker

Reviewed By: samparker

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73970

(cherry picked from commit 8ba2b628)