[BranchFolding] Add an optional target hook to skip branch folding when it's unsafe

[csstormq]

Assume the indirect branch implementation is as follows:

mov r0.lo BB0.label_lo
mov r0.hi BB0.label_hi
add r2 r1 r0
jr r2

Here, the base address and offset are stored in r0  and r1 respectively, with r2 = r1 + r0 as the jump target. Analyzing the target of the indirect branch jr  is often challenging, particularly when register r2 is spilled.

When basic block BB0 is empty, BranchFolding may optimize it, causing a dangling label. To address this, the target can override  isMBBSafeToBranchFolding  and return  false.

This issue was found in an out-of-tree target, hence the lack of testcase in this commit.