[SimplifyCFG] Thread branches on same condition in more cases (PR54980)

SimplifyCFG implements basic jump threading, if a branch is
performed on a phi node with constant operands. However,
InstCombine canonicalizes such phis to the condition value of a
previous branch, if possible. SimplifyCFG does support this as
well, but only in the very limited case where the same condition
is used in a direct predecessor -- notably, this does not include
the common diamond pattern (i.e. two consecutive if/elses on the
same condition).

This patch extends the code to look back a limited number of
blocks to find a branch on the same value, rather than only
looking at the direct predecessor.

Fixes https://github.com/llvm/llvm-project/issues/54980.

Differential Revision: https://reviews.llvm.org/D124159