Extend AsmMatcher token literal matching to allow aliasing.

Extend AsmMatcher token literal matching to allow aliasing.

For example, ARM allows:
    vmov.u32 s4, #0  -> vmov.i32, #0
'u32' is a more specific designator for the 32-bit integer type specifier
and is legal for any instruction which accepts 'i32' as a datatype suffix.

We want to say,
    def : TokenAlias<".u32", ".i32">;

This works by marking the match class of 'From' as a subclass of the
match class of 'To'.

rdar://10435076

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145992 91177308-0d34-0410-b5e6-96231b3b80d8