Added #sint8: and #unit8: constructors to `LLVMConstant` class

Added LLVMValue>>initilizer: to allow constanv value fixups. This is useful in cases when the value of the global variable is not known at the time it's created - but it must be known before a machine code is emmited, indeed.

New utility methods to create constant pointers

Fixed LLVMIRBuilder>>if:then:else: for nested ifs and complex branches.

Added LLVMModule>>verify and LLVMFunction>>verify

Renamed #store:_: to #store:at: which reads better

Tests: Added LLBMIRBuilderTests

Added 'block form' of #if:then:else: for easier use A 'block form' allows one to use it as in following example: asm if: cond then: [ asm ... ] else: [ asm ... ]. adm ... This means that user of LLVMIRBuilder need not fiddle about basic blocks and their proper linking for generate a simple condition.

Added support for PHI nodes. Even though manual use of these is discouraged, this may be very handy when one needs to generate `cond ? tval : fval` kind of expression. In some cases one may not use `select` instructions is nay compile to CMOV on Intel which has its issues and may lead to SEGV.

Fixed finalization of MCJIT memory manager / execution engine If options specifies a custom MM, must unregister it from finalization as the disposal of LLVMExecutionEngine disposes the MM itself. If we won't unregister it, it will be disposed twice.