[vm/compiler] Move BlockBuilder from graph_intrinsifier.cc to a header.

This helper class can be used for building IL in SSA form. It was
previously used to build intrinsic IL, but it can also be used to
build IL for tests.

This move required introducing a constant that describes offset
of the last parameter from SP value on entry (kLastParamFrom

This CL also changes the meaning of ParameterInstr(index, SPREG) to be
[SP + index + kLastParamSlotFromEntrySp], where previously it was
something like [SP + (index - 1) + kLastParamSlotFromEntrySp].

Change-Id: Ib75ecb3a94f9cde74d6f91028d3cbea8f5e4bc42
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/98336
Commit-Queue: Vyacheslav Egorov <vegorov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>