Add avg_{up,down}_int{8,16} routines to stdlib

These compute the average of two given values, rounding up and down,
respectively, if the result isn't exact.  When possible, these are
mapped to target-specific intrinsics (PADD[BW] on IA and VH[R]ADD[US]
on NEON.)

A subsequent commit will add pattern-matching to generate calls to
these intrinsincs when the corresponding patterns are detected in the
IR.)

opt.cpp

@@ -4343,6 +4343,14 @@ char MakeInternalFuncsStaticPass::ID = 0;
 bool
 MakeInternalFuncsStaticPass::runOnModule(llvm::Module &module) {
     const char *names[] = {
+        "__avg_up_uint8",
+        "__avg_up_int8",
+        "__avg_up_uint16",
+        "__avg_up_int16",
+        "__avg_down_uint8",
+        "__avg_down_int8",
+        "__avg_down_uint16",
+        "__avg_down_int16",
         "__fast_masked_vload",
         "__gather_factored_base_offsets32_i8", "__gather_factored_base_offsets32_i16",
         "__gather_factored_base_offsets32_i32", "__gather_factored_base_offsets32_i64",