[AMDGPU] Use correct number of bits needed for div/rem shrinking (#80622)

There was an error where dividend of type i64 and actual used number of
bits of 32 fell into path that assumes only 24 bits being used. Check
that AtLeast field is used correctly when using computeNumSignBits and
add necessary extend/trunc for 32 bits path.

Regolden and update testcases.

@jrbyrnes @bcahoon @arsenm @rampitec