[ARM] GlobalISel: Fix extended stack operands

Fix a crash when trying to extend a value passed as a sign- or
zero-extended stack parameter. The cause of the crash was that we were
setting the size of the loaded value to 32 bits, and then tyring to
extend again to 32 bits.

This patch addresses the issue by also introducing a G_TRUNC after the
load. This will leave the unused bits to their original values set by
the caller, while being consistent about the types. For values that are
not extended, we just use a smaller load.

llvm-svn: 301531