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
