[openmp][nfc] Refactor GridValues

Remove redundant fields and replace pointer with virtual function

Of fourteen fields, three are dead and four can be computed from the
remainder. This leaves a couple of currently dead fields in place as
they are expected to be used from the deviceRTL shortly. Two of the
fields that can be computed are only used from codegen and require a
log2() implementation so are inlined into codegen instead.

This change leaves the new methods in the same location in the struct
as the previous fields for convenience at review.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D108380