[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[Mel Chen via llvm-commits]

Mel-Chen wrote:

> Sorry for the ignorant question in advance - I am not familiar with the mechanics of vectorization.
> 
> How does this work with FP values? If the undef value happens to be NaN, then do we care that the comparison will return false (regardless of whether the initial value was updated or not by the vector loop)?

@vzakhari Both AnyOf and FindLastIV do not support floating-point types so far.

https://github.com/llvm/llvm-project/pull/132691