[TLI] ReplaceWithVecLib: drop Instruction support (#94365)

Refactor the pass to only support `IntrinsicInst` calls.

`ReplaceWithVecLib` used to support instructions, as AArch64 was using
this pass to replace a vectorized frem instruction to the fmod vector
library call (through TLI).

As this replacement is now done by the codegen (#83859), there is no
need for this pass to support instructions.

Additionally, removed 'frem' tests from:
- AArch64/replace-with-veclib-armpl.ll
- AArch64/replace-with-veclib-sleef-scalable.ll
- AArch64/replace-with-veclib-sleef.ll

Such testing is done at codegen level:
- #83859