[GlobalISel] Implement computeNumSignBits for G_ASSERT_SEXT

Same implementation as G_SEXT_INREG.

Add a testcase to combine-sext-inreg for a concrete example, and a testcase
to KnownBitsTest.

Differential Revision: https://reviews.llvm.org/D96897

GitOrigin-RevId: 26fb036559d05b601a585c751177d6ec367fb65b
3 files changed