[openmp][AMDGPU] - Correct getNumberOfBlocks calculation.
This patch fixes the 6 amdgpu buildbot lit test failures
introduced by https://reviews.llvm.org/D135444.
libomptarget :: amdgcn-amd-amdhsa :: mapping/reduction_implicit_map.cpp
libomptarget :: amdgcn-amd-amdhsa :: offloading/cuda_no_devices.c
libomptarget :: amdgcn-amd-amdhsa :: offloading/target-teams-atomic.c
libomptarget :: amdgcn-amd-amdhsa-LTO :: mapping/reduction_implicit_map.cpp
libomptarget :: amdgcn-amd-amdhsa-LTO :: offloading/cuda_no_devices.c
libomptarget :: amdgcn-amd-amdhsa-LTO :: offloading/target-teams-atomic.c
Reviewed By: jhuber6
Differential Revision: https://reviews.llvm.org/D137261