R600/SI: Teach SIFoldOperands to split 64-bit constants when folding

This allows folding of sequences like:

s[0:1] = s_mov_b64 4
v_add_i32 v0, s0, v0
v_addc_u32 v1, s1, v1

into

v_add_i32 v0, 4, v0
v_add_i32 v1, 0, v1

llvm-svn: 225369