[RISCV] Fix assertion failure when using -fstack-clash-protection (#135248)
We can't assume MBBI is still pointing at MBB if we've already expanded
a probe. We need to re-query the MBB from MBBI. Fixes #135206
Co-authored-by: Craig Topper <craig.topper@sifive.com>
(cherry picked from commit b3d2dc321c5c78b7204696afe07fe6ef3375acfd)