Generate lookup tables instead of jump tables where possible.

Motivation

Sometimes we end up with Cmm code of this kind:

       uXSY: // global
           switch [0 .. 22] _cXRy::I64 {
               case 0 : goto cXM8;
               case 1 : goto cXMc;
               ...
               case 17, 18, 19, 20, 21 : goto cXMQ;
               case 22 : goto cXNy;
           }
       cXNy: // global
           _sOUk::P64 = lvl1345_rNxA_closure+1;
           goto sOUi;
       cXMQ: // global
           _sOUk::P64 = lvl1344_rNxz_closure+1;
           goto sOUi;

       ... a few more of this kind

       cXM8: // global
           _sOUk::P64 = lvl1339_rNxu_closure+1;
           goto sOUi;
       cXM4: // global
           _sOUk::P64 = lvl1338_rNxt_closure+1;
           goto sOUi;

(Example is from GHC's lexer)

The issue with the current code

Currently this is compiled into the following essential pieces:

#A indirect jump, branching on the value
_uXSY:
	jmp *_nXUl(,%rbx,8)

# The jump table
.section .rdata
_nXUl:
	.quad	_cXM8
	.quad	_cXMc
        ...
	.quad	_cXMQ
	.quad	_cXNy

#And the jump target blocks
_cXMQ:
	movl $lvl1344_rNxz_closure+1,%ebx
_nXUs:
	movq %rax,%r14
	movq %rbx,%rax
	jmp _sOUi
_cXNy:
	movl $lvl1345_rNxA_closure+1,%ebx
_nXUr:
	movq %rax,%r14
	movq %rbx,%rax
	jmp _sOUi
#and here would be 6 more.

I like how this example is made even better since we accidentally duplicate the register shuffling in each of those blocks.

Proposal

However instead the code could be simply replaced with this

       uXSY: // global
           _sOUk::P64 = P64[table+_cXRy];
           goto sOUi;

// + The lookup table

Which is just so much better in all kinds of ways.