[RISCV][NFC] Rework CHECK lines in rvi-aliases-valid.s

Previously CHECK prefixes weren't defined that can be used to check _only_ the 
InstPrinter output when generating .s from llvm-mc, or that check _only_ the 
output after passing the generated object through objdump. This means we can't 
write useful checks for instructions that reference symbols.

Instead, use:
CHECK-S                 Match the .s output with aliases enabled
CHECK-S-NOALIAS         Match the .s output with aliases disabled
CHECK-OBJ               Match the objdumped object output with aliases enabled
CHECK-OBJ-NOALIAS       Match the objdumped object output with aliases enabled
CHECK-S-OBJ             Match both the .s and objdumped object output with
                        aliases enabled
CHECK-S-OBJ-NOALIAS     Match both the .s and objdumped object output with
                        aliases disabled

While we're at it, use whitespace consistently within this file.

llvm-svn: 341050