[sgen] Performance improvement for ms_block_size computations

[sgen] Performance improvement for ms_block_size computations

This reduces regressions for switching from MS_BLOCK_SIZE compile time constant
to ms_block_size variable by 85% on the tests where this was showing up. In the
future we should implement a block allocator, which would, amongst other benefits,
enable us to remove the block-size/page-size dependency.