miri: Shl/Shr should throw UB on too large shift

The LLVM docs for shl say

If op2 is (statically or dynamically) equal to or larger than the number of bits in op1, this instruction returns a poison value.

However, the CTFE/Miri implementation of shl/shr currently just truncates the shift amount. In rustc-generated MIR this can never happen since there always is a check being emitted, but we should still make sure to implement all UB checks for this operation correctly – and the simd_shl intrinsic also exposes direct access to this operation, making it possible to trigger this UB. (I plan to add tests for this.)

I just hope we didn’t miss any other MIR operations which compile ti LLVM arithmetic operations that can produce poison/undef/UB… (we do check for div-by-zero, of course). Cc @rust-lang/wg-llvm