transforms: Fix -Wimplicit-fallthrough warning
transforms: Fix -Wimplicit-fallthrough warning

Fixes:
PrepareOverflows.cpp: In member function ‘void {anonymous}::PrepareOverflows::handleIntrinsic(llvm::IntrinsicInst*)’:
PrepareOverflows.cpp:66:34: warning: this statement may fall through [-Wimplicit-fallthrough=]
   66 |             handleSBinOpIntrinsic(II, Instruction::BinaryOps::Add);
      |             ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PrepareOverflows.cpp:67:11: note: here
   67 |           case Intrinsic::ssub_with_overflow:
      |           ^~~~
PrepareOverflows.cpp:68:34: warning: this statement may fall through [-Wimplicit-fallthrough=]
   68 |             handleSBinOpIntrinsic(II, Instruction::BinaryOps::Sub);
      |             ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PrepareOverflows.cpp:69:11: note: here
   69 |           case Intrinsic::smul_with_overflow:
      |           ^~

staticafi/symbiotic,staticafi/symbiotic,staticafi/symbiotic,staticafi/symbiotic