[mips][msa] Range adjustment for ldi_b builtin function operand

Reasoning behind this change was allowing the function to accept all values
from range [-128, 255] since all of them can be encoded in an 8bit wide
value.
This differs from the prior state where only range [-128, 127] was accepted,
where values were assumed to be signed, whereas now the actual
interpretation of the immediate is deferred to the consumer as required.

Patch by Stefan Maksimovic.

Differential Revision: https://reviews.llvm.org/D31082

llvm-svn: 299229