[AMDGPU] Do not generate S_CMP_LG_U64 on gfx7

S_CMP_LG_U64 was added in gfx8 and is guarded by hasScalarCompareEq64().

Rewrite S_CMP_LG_U64 to S_OR_B32 + S_CMP_LG_U32 for targets that
do not support 64-bit scalar compare.

Differential Revision: https://reviews.llvm.org/D89536