…mentation (aws#2227)

- Fix instruction that caused out-of-bound read in the input reading of
the 16x loop (which processes a batch of 16 blocks of AES, 1 block = 16
bytes). This was triggered on lengths that are in the range 
[16*k * (16 bytes), (16*k +3)* (16 bytes)-1], k = 1, 2, ... 
The instruction was reading up to 3*16 bytes beyond the input length bound.

- The fix was inspired by the 8x loop in
https://github.com/aws/aws-lc/blob/becf5785c131012bb5a64f3da6cdb117ddc0f431/crypto/fipsmodule/aes/asm/aesni-xts-avx512.pl#L2544

- The existing unit tests cover those cases but there were no explicit
memory protections and ASAN doesn't instrument assembly code to check
for out-of-bound reads even when the subsequent memory is explicitly
poisoned.
 
### Call-outs:
N/A

### Testing:
On c6i, without the fix, the unit test segfaults
```
./crypto/crypto_test "--gtest_filter=XTSTest.*"
Note: Google Test filter = XTSTest.*
[==========] Running 4 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 4 tests from XTSTest
[ RUN      ] XTSTest.TestVectors
Segmentation fault (core dumped)
```
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