[OpenMP][Offloading] Replace all calls to `isSPMDMode` with `__kmpc_is_spmd_exec_mode`

In our ongoing work, we are using `AbstractAttributor` to deduct execution model
of device functions, and potententially remove unnecessary function calls to
`__kmpc_is_spmd_exec_mode`. In current device runtime, we have mixed use of
`isSPMDMode` and `__kmpc_is_spmd_exec_mode`, but in fact in `__kmpc_is_spmd_exec_mode`
it simply calls `isSPMDMode`. Since all functions starting with `__kmpc` is C
function, which doesn't have things like name mangling. It is more optimization
friendly. In this patch, we simply replaced all calls to `isSPMDMode` with
`__kmpc_is_spmd_exec_mode` to pave the way for the optimization.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D105211