Refactor __kmp_execute_tasks_template function

Refactored __kmp_execute_tasks_template to shorten and remove code redundancy.
The original code for __kmp_execute_tasks_template was very redundant with
large sections of repeated code that needed to be kept consistent, and goto
statements that made the control flow difficult to discern. This refactoring
removes all gotos and redundancy.

Patch by Terry Wilmarth

Differential Revision: http://reviews.llvm.org/D20879

llvm-svn: 272286