[MC] Store operand info immediately after the TargetInsts table. NFC.

This shrinks MCInstrDesc (and hence the whole TargetInsts table) because
we can store a 16-bit offset value to access the operands info, instead
of a pointer. This also reduces the number of relocs that need to be
applied when LLVM is compiled as position-independent code.

Differential Revision: https://reviews.llvm.org/D142219