[AArch64][SVE] Remove assertion/range check for i16 values during immediate selection

The assertion can fail in some cases when an i16 constant is promoted
to i32.

e.g. in the added test case the value `i16 -32768` is within the range
of i16 but the assert fails when the constant is promoted to positive
`i32 32768` by an earlier call to DAG.getConstant().

Differential Revision: https://reviews.llvm.org/D107880

Change-Id: I2f6179783cbc9630e6acab149a762b43c65664de