TableGen/GlobalISel: Fix pattern matching of immarg literals

For arguments that are not expected to be materialized with
G_CONSTANT, this was emitting predicates which could never match. It
was first adding a meaningless LLT check, which would always fail due
to the operand not being a register.

Infer the cases where a literal should check for an immediate operand,
instead of a register This avoids needing to invent a special way of
representing timm literal values.

Also handle immediate arguments in GIM_CheckLiteralInt. The comments
stated it handled isImm() and isCImm(), but that wasn't really true.

This unblocks work on the selection of all of the complicated AMDGPU
intrinsics in future commits.