Simplify arena_slab_regind().

Rewrite arena_slab_regind() to provide sufficient constant data for
the compiler to perform division strength reduction.  This replaces
more general manual strength reduction that was implemented before
arena_bin_info was compile-time-constant.  It would be possible to
slightly improve on the compiler-generated division code by taking
advantage of range limits that the compiler doesn't know about.

test/unit/slab.c

@@ -0,0 +1,35 @@
+#include "test/jemalloc_test.h"
+
+TEST_BEGIN(test_arena_slab_regind)
+{
+	szind_t binind;
+
+	for (binind = 0; binind < NBINS; binind++) {
+		size_t regind;
+		extent_t slab;
+		const arena_bin_info_t *bin_info = &arena_bin_info[binind];
+		extent_init(&slab, NULL, mallocx(bin_info->slab_size,
+		    MALLOCX_LG_ALIGN(LG_PAGE)), bin_info->slab_size, 0, 0, true,
+		    false, true, true);
+		assert_ptr_not_null(extent_addr_get(&slab),
+		    "Unexpected malloc() failure");
+		for (regind = 0; regind < bin_info->nregs; regind++) {
+			void *reg = (void *)((uintptr_t)extent_addr_get(&slab) +
+			    (bin_info->reg_size * regind));
+			assert_zu_eq(arena_slab_regind(&slab, binind, reg),
+			    regind,
+			    "Incorrect region index computed for size %zu",
+			    bin_info->reg_size);
+		}
+		free(extent_addr_get(&slab));
+	}
+}
+TEST_END
+
+int
+main(void)
+{
+
+	return (test(
+	    test_arena_slab_regind));
+}