Rather than allow arbitrary shift+trunc, only concern ourselves
with low and high parts. This is all that was being used anyway.
Backports commit 609ad70562793937257c89d07bf7c1370b9fc9aa from qemu
The op is sometimes named trunc_shr_i32 and sometimes trunc_shr_i64_i32,
and the name in the README doesn't match the name offered to the
frontends.
Always use the long name to make it clear it is a size changing op.
Backports commit 0632e555fc4d281d69cb08d98d500d96185b041f from qemu