[RISCV] Allow call pseudoinstruction to be used to call a function name that coincides with a register name

Previously `call zero`, `call f0` etc would fail. This leads to compilation 
failures if building programs that define functions with those names and using 
-save-temps.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330846 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed