itx: cancel 16 out of '(x * 2896) >> 12' to avoid integer overflows

Fixes an integer overflow in inv_dct4_1d with
clusterfuzz-testcase-dav1d_fuzzer-5634807321591808 and in inv_adst16_1d
with clusterfuzz-testcase-dav1d_fuzzer-5761827623927808. Credits to oss-fuzz.
12 jobs for master in 4 minutes and 55 seconds (queued for 2 seconds)
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