Keep vp8 sixtap read within bounds

When filtering it needs 6 pixels: 2 prior to the source, the source, and
3 after the source.

When filtering 16 wide, that means 21. To accomplish this the SSE2 reads
[-2] to [5], [6] to [13], and [14] to [21], a total of 24 bytes (reading
in groups of 8 is easy)

The filter then shifts this last set to the top half of the register and
uses 'or' to combine it with the previous set.

Valgrind detected an issue reading pixels [19], [20] and [21]:
Address 0x7f581c2 is 434 bytes inside a block of size 441 alloc'd

Note: we only need pixels [16], [17], and [18] as context for [15].

To fix this, it now reads 8 bytes starting at [11], which re-loads [11]
through [13], but stops at [18] and does not over-read any values.

This is shifted by 5 and 'or'd with xmm1. Although the lower bits are
not cleared, they overlap directly with [11] through [13], so 'or'
produces the correct results.

Change-Id: I0c89c03afa660fc9b0108ac055d7bd403e493320

vp8/common/x86/subpixel_sse2.asm

@@ -181,8 +181,12 @@ sym(vp8_filter_block1d16_h6_sse2):
         movq        xmm3,       MMWORD PTR [rsi - 2]
         movq        xmm1,       MMWORD PTR [rsi + 6]
 
-        movq        xmm2,       MMWORD PTR [rsi +14]
-        pslldq      xmm2,       8
+        ; Load from 11 to avoid reading out of bounds.
+        movq        xmm2,       MMWORD PTR [rsi +11]
+        ; The lower bits are not cleared before 'or'ing with xmm1,
+        ; but that is OK because the values in the overlapping positions
+        ; are already equal to the ones in xmm1.
+        pslldq      xmm2,       5
 
         por         xmm2,       xmm1
         prefetcht2  [rsi+rax-2]