[AArch64] Rename some timm predicates for consistency. NFC.

timm isn't the common case, and TImmLeafs should make it clear what
they are.  We're adding a plain ImmLeaf for 0_65535, so rename
i64_imm0_65535 to timm64_0_65535, and imm32_0_7 to timm32_0_7.