[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299229
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