[VE] Optimize trunc related instructions

[VE] Optimize trunc related instructions

Change to not generate truncate instructions if all use of a truncate
operation don't care about higher bits.  For example, an i32 add
instruction doesn't care about higher 32 bits in 64 bit registers.
Updates regression tests also.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D85418