[llvm] [AMDGPU] Rework getDivNumBits API (PR #119768)

[via llvm-commits]

choikwa wrote:

>From debugging, I've observed that in shrinkDivRem64, 
```
int NumDivBits = getDivNumBits(I, Num, Den, 32 /*AtLeast*/, IsSigned);
if (NumDivBits == -1)                                                                                                                                                                                                     
  return nullptr; 
```
It's rejecting the case where operands of Num or Den have <=32 DivNumBits but the SignNumBits are also <32. Such a case arises when i.e. Num or Den is ` %1 = ashr i48 %x, 24` or when source's number of scalar bits are in (64,32) since AtLeast logic only depends on number of sign bits.

https://github.com/llvm/llvm-project/pull/119768