[BOLT][RISCV] Handle CIE's produced by GNU as

[mtvec]

On RISC-V, GNU as produces the following initial instruction in CIE's:

DW_CFA_def_cfa_register: r2

While I believe it is technically illegal to use this instruction without first using a DW_CFA_def_cfa (since the offset is undefined), both readelf and llvm-dwarfdump accept this and implicitly set the offset to 0.

In BOLT, however, this triggers an assert (in CFISnapshot::advanceTo) as it (correctly) believes the offset is not set. This patch fixes this by setting the offset to 0 whenever executing DW_CFA_def_cfa_register while the offset is undefined.

Note that this is probably the simplest workaround but it has a downside: while emitting CFI start, we check if the initial instructions are contained within MCAsmInfo::getInitialFrameState and omit them if they are. This will not be true for GNU CIE's (since they differ from LLVM's) which causes an unnecessary DW_CFA_def_cfa_register to be emitted.

While technically correct, it would probably be better to replace the GNU CIE with the one used by LLVM to avoid this situation. This would solve the problem this patch solves while also preventing unnecessary CFI instructions. However, this is a bit trickier to implement correctly so I propose to keep this for a later time.

Note on testing: the test creates a simple function with three basic blocks and forces the CFI state of the last one to be different from the others using an arbitrary CFI instruction. Then,
--reorder-blocks=reverse is used to force CFISnapshot::advanceTo to be called. This causes an assert on the current main branch.

[rafaelauler]

While technically correct, it would probably be better to replace the GNU CIE with the one used by LLVM to avoid this situation.

I think the extra CFI being inserted is a small price to pay for compatibility with GNU binaries. I wouldn't worry about it.