x86/convolve.h: replace while w/if for w < 16

in non-hbd configurations; any high-bitdepth changes will be done in a
follow-up

Change-Id: Ia74e30971b744c1faab68c92fdeda1a053988c77

vpx_dsp/x86/convolve.h

diff --git a/vpx_dsp/x86/convolve.h b/vpx_dsp/x86/convolve.h
index df6642e..a26385e 100644
--- a/vpx_dsp/x86/convolve.h
+++ b/vpx_dsp/x86/convolve.h
@@ -45,27 +45,20 @@
       dst += 16; \
       w -= 16; \
     } \
-    while (w >= 8) { \
+    if (w == 8) { \
       vpx_filter_block1d8_##dir##8_##avg##opt(src_start, \
                                               src_stride, \
                                               dst, \
                                               dst_stride, \
                                               h, \
                                               filter); \
-      src += 8; \
-      dst += 8; \
-      w -= 8; \
-    } \
-    while (w >= 4) { \
+    } else if (w == 4) { \
       vpx_filter_block1d4_##dir##8_##avg##opt(src_start, \
                                               src_stride, \
                                               dst, \
                                               dst_stride, \
                                               h, \
                                               filter); \
-      src += 4; \
-      dst += 4; \
-      w -= 4; \
     } \
   } else { \
     while (w >= 16) { \
@@ -79,27 +72,20 @@
       dst += 16; \
       w -= 16; \
     } \
-    while (w >= 8) { \
+    if (w == 8) { \
       vpx_filter_block1d8_##dir##2_##avg##opt(src, \
                                               src_stride, \
                                               dst, \
                                               dst_stride, \
                                               h, \
                                               filter); \
-      src += 8; \
-      dst += 8; \
-      w -= 8; \
-    } \
-    while (w >= 4) { \
+    } else if (w == 4) { \
       vpx_filter_block1d4_##dir##2_##avg##opt(src, \
                                               src_stride, \
                                               dst, \
                                               dst_stride, \
                                               h, \
                                               filter); \
-      src += 4; \
-      dst += 4; \
-      w -= 4; \
     } \
   } \
 }