[libc] Make `BigInt` bitwise shift consistent with regular integral semantics. (#87874)

This patch removes the test for cases where the shift operand is greater
or equal to the bit width of the number. This is done for two reasons,
first it makes `BigInt` consistent with regular integral bitwise shift
semantics, and second it makes the shift operation faster. The shift
operation is on the critical path for `exp` and `log` operations, see
https://github.com/llvm/llvm-project/pull/86137#issuecomment-2034133868.