[TableGen] Increase the number of supported decoder fix-ups.

The vast number of added instructions for SVE causes TableGen to fail with an assertion:

  Assertion `Delta < 65536U && "disassembler decoding table too large!"'

This patch increases the number of supported decoder fix-ups.

Reviewers: dmgreen, stoklund, petpav01

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D48937

llvm-svn: 336334