[TLI] ReplaceWithVecLib: drop Instruction support

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.l

Such testing is done at codegen level:
- #83859