In CSKYConstantIslands, when fix up an unconditional branch(CSKY::BR32) whose destination is
too far away to fit in its displacement field, and if the R15(LR) register has been
spilled in the prologue, then we can use BSR to implement a far jump. So we need estimate function
size, and spill R15(LR) when the function size >= unconditional branch(CSKY::BR32) can reach.
EstimateFunctionSizeInBytes function adds up all instructions and constant pool entries(each entry is 4 bytes).
Ooops. It constructs codegen infra and provide only basic code to generate first add instruction successfully.
Differential Revision: https://reviews.llvm.org/D112206
