Improvements to machine_hierarchy code for re-sizing

These changes include:
 1) Machine hierarchy now uses the base_num_threads field to indicate the 
    maximum number of threads the current hierarchy can handle without a resize.
 2) In __kmp_get_hierarchy, we need to get depth after any potential resize
    is done.
 3) Cleanup of hierarchy resize code to support 1 above.

Differential Revision: http://reviews.llvm.org/D14455

llvm-svn: 252475