[FuzzMutate] Correct type cast and add unit test for FCmp

This revision fixes an incorrect type cast from Instruction to ICmpInstr, which should have been to FCmpInstr instead. It turns out that StrategiesTest.cpp was missing a test case for InstModificationIRStrategy and FCmp, which is also now implemented in this revision. After this revision, [[ https://reviews.llvm.org/D148854 | llvm-stress in D148854 ]] no longer crashes randomly.

Reviewed By: Peter

Differential Revision: https://reviews.llvm.org/D148972