cranelift: Deduplicate ExtName for avg_round tests

cranelift/filetests/filetests/runtests/simd-arithmetic.clif

@@ -174,19 +174,19 @@ block0(v0: f32x4):
 }
 ; run: %fabs_f32x4([0x0.0 -0x1.0 0x2.0 -0x3.0]) == [0x0.0 0x1.0 0x2.0 0x3.0]
 
-function %average_rounding_i8x16(i8x16, i8x16) -> i8x16 {
+function %avg_round_i8x16(i8x16, i8x16) -> i8x16 {
 block0(v0: i8x16, v1: i8x16):
     v2 = avg_round v0, v1
     return v2
 }
-; run: %average_rounding_i8x16([0 0 0 1 42 19 -1 0xff 5 0 0 0 1 42 19 -1], [0 1 2 4 42 18 -1 0 10 0 1 2 4 42 18 -1]) == [0 1 1 3 42 19 -1 0x80 8 0 1 1 3 42 19 -1]
+; run: %avg_round_i8x16([0 0 0 1 42 19 -1 0xff 5 0 0 0 1 42 19 -1], [0 1 2 4 42 18 -1 0 10 0 1 2 4 42 18 -1]) == [0 1 1 3 42 19 -1 0x80 8 0 1 1 3 42 19 -1]
 
-function %average_rounding_i16x8(i16x8, i16x8) -> i16x8 {
+function %avg_round_i16x8(i16x8, i16x8) -> i16x8 {
 block0(v0: i16x8, v1: i16x8):
     v2 = avg_round v0, v1
     return v2
 }
-; run: %average_rounding_i16x8([0 0 0 1 42 19 -1 0xffff], [0 1 2 4 42 18 -1 0]) == [0 1 1 3 42 19 -1 0x8000]
+; run: %avg_round_i16x8([0 0 0 1 42 19 -1 0xffff], [0 1 2 4 42 18 -1 0]) == [0 1 1 3 42 19 -1 0x8000]
 
 function %iabs(i32x4) -> i32x4 {
 block0(v0: i32x4):