[SelectionDAG][X86] Support inline assembly returning an mmx register into a...

[SelectionDAG][X86] Support inline assembly returning an mmx register into a type with fewer than 64 bits.

It's possible to use the 'y' mmx constraint with a type narrower than 64-bits.

This patch supports this by bitcasting the mmx type to 64-bits and then
truncating to the desired type.

There are probably other missing type combinations we need to support, but this
is the case we have a bug report for.

Fixes PR41748.

Differential Revision: https://reviews.llvm.org/D61582

llvm-svn: 360069