[llvm] [EarlyIfConversion] Extend data dependent analysis across multiple blocks (PR #180623)

[Jonathan Cohen via llvm-commits]

jcohen-apple wrote:

This is an intermediate step before landing a larger optimization for converting groups of cascading branches, which relies on this data dependence analysis.

The actual change itself is between performance neutral to slightly positive across several internal benchmark suites.

We see a +7% overall conversion increase of both diamonds and triangles, and between 0.5-1% reduction in code size on average. 

No measurable compile time impact using test-suite `ctmark` or in our internal workloads. 

https://github.com/llvm/llvm-project/pull/180623