[X86CmovConversion] Make heuristic for optimized cmov depth more conservative (PR44539)

Fix/workaround for https://bugs.llvm.org/show_bug.cgi?id=44539.
As discussed there, this pass makes some overly optimistic
assumptions, as it does not have access to actual branch weights.

This patch makes the computation of the depth of the optimized cmov
more conservative, by assuming a distribution of 75/25 rather than
50/50 and placing the weights to get the more conservative result
(larger depth). The fully conservative choice would be
std::max(TrueOpDepth, FalseOpDepth), but that would break at least
one existing test (which may or may not be an issue in practice).

Differential Revision: https://reviews.llvm.org/D74155