[TableGen] Extend OPC_ExtractField/OPC_CheckField start value widths.

[nvjle]

Both OPC_ExtractField and OPC_CheckField are currently defined to take an unsigned 8-bit start value. On some architectures with long instruction words, this value can silently overflow, resulting in a bad decoder table.

This patch changes each to take a ULE128B-encoded start value instead. Additionally, a range assertion is added for the 8-bit length to prominently notify a user in case that field ever overflows.

This problem isn't currently exposed upstream since all in-tree targets use small instruction words (i.e., bitwidth <= 64 bits). It does show up in at least one downstream target with instructions > 64 bits long.

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[nvjle]

I hope one of @jyknight @jmolloy @0x59616e (based on recent commits to the same area) might review this small patch, thank you.

[jyknight]

LGTM, thanks; merging.

[nvjle]

LGTM, thanks; merging.

Thank you @0x59616e and @jyknight, greatly appreciated.