AMDGPU: Delete probably wrong constant folding of expm1

[arsenm]

Summary

It's not really correct to implement this as exp(x) - 1, it was maybe
OK for the restricted float-as-double case handled here. There's not a
strong reason to special case it with the host function, as the
implementation naturally constant folds anyway. InstSimplify can fully
handle everything in it, so just running the inliner alone with a
constant argument produces the fully constant folded result. This also
had no tests.

[arsenm]

b0f4b6587a8e1044f6e4e1e117cc166586678ee5

llvm/lib/Target/AMDGPU/AMDGPULibCalls.cpp

bool AMDGPULibCalls::evaluateScalarMathFunc(const FuncInfo &FInfo,
   case AMDGPULibFunc::EI_EXP2:
     Res0 = pow(2.0, opr0);
     return true;
 
   case AMDGPULibFunc::EI_EXP10:
     Res0 = pow(10.0, opr0);
     return true;
 
-  case AMDGPULibFunc::EI_EXPM1:
-    Res0 = exp(opr0) - 1.0;
-    return true;
-
   case AMDGPULibFunc::EI_LOG:
     Res0 = log(opr0);
     return true;
 
   case AMDGPULibFunc::EI_LOG2:
     Res0 = log(opr0) / log(2.0);
     return true;